diff --git a/.circleci/config.yml b/.circleci/config.yml
index 2d8f38a76e..43f18b010b 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -68,7 +68,7 @@ jobs:
         name: "Set up environment"
         command: |
           source ./.circleci/setup.bash && setup php7
-          wget https://github.com/phpstan/phpstan/releases/download/0.11.4/phpstan.phar
+          wget https://github.com/phpstan/phpstan/releases/download/0.11.5/phpstan.phar
     - run:
         name: "Static analysis"
         command: ./do qa:phpstan
diff --git a/lib/API/JSON/v1/CustomFields.php b/lib/API/JSON/v1/CustomFields.php
index 3ec3377b82..f660cd4c4b 100644
--- a/lib/API/JSON/v1/CustomFields.php
+++ b/lib/API/JSON/v1/CustomFields.php
@@ -27,14 +27,14 @@ class CustomFields extends APIEndpoint {
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : null);
     $custom_field = CustomField::findOne($id);
-    if ($custom_field === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This custom field does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($custom_field instanceof CustomField) {
       $custom_field->delete();
 
       return $this->successResponse($custom_field->asArray());
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This custom field does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -44,22 +44,20 @@ class CustomFields extends APIEndpoint {
 
     if (!empty($errors)) {
       return $this->badRequest($errors);
-    } else {
-      return $this->successResponse(
-        CustomField::findOne($custom_field->id)->asArray()
-      );
     }
+    $custom_field = CustomField::findOne($custom_field->id);
+    if(!$custom_field instanceof CustomField) return $this->errorResponse();
+    return $this->successResponse($custom_field->asArray());
   }
 
   function get($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : null);
     $custom_field = CustomField::findOne($id);
-    if ($custom_field === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This custom field does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($custom_field instanceof CustomField) {
       return $this->successResponse($custom_field->asArray());
     }
+    return $this->errorResponse(array(
+      APIError::NOT_FOUND => WPFunctions::get()->__('This custom field does not exist.', 'mailpoet')
+    ));
   }
 }
diff --git a/lib/API/JSON/v1/Forms.php b/lib/API/JSON/v1/Forms.php
index daaddb7e51..adacab98c2 100644
--- a/lib/API/JSON/v1/Forms.php
+++ b/lib/API/JSON/v1/Forms.php
@@ -37,13 +37,12 @@ class Forms extends APIEndpoint {
   function get($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
-    if ($form === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($form instanceof Form) {
       return $this->successResponse($form->asArray());
     }
+    return $this->errorResponse(array(
+      APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
+    ));
   }
 
   function listing($data = array()) {
@@ -111,13 +110,12 @@ class Forms extends APIEndpoint {
     $form = Form::createOrUpdate($data);
     $errors = $form->getErrors();
 
-    if (!empty($errors)) {
-      return $this->badRequest($errors);
-    } else {
-      return $this->successResponse(
-        Form::findOne($form->id)->asArray()
-      );
+    if (empty($errors)) {
+      $form = Form::findOne($form->id);
+      if(!$form instanceof Form) return $this->errorResponse();
+      return $this->successResponse($form->asArray());
     }
+    return $this->badRequest($errors);
   }
 
   function previewEditor($data = array()) {
@@ -139,19 +137,16 @@ class Forms extends APIEndpoint {
   function exportsEditor($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
-    if ($form === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($form instanceof Form) {
       $exports = Util\Export::getAll($form->asArray());
       return $this->successResponse($exports);
     }
+    return $this->errorResponse(array(
+      APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
+    ));
   }
 
   function saveEditor($data = array()) {
-    $id = (isset($data['id']) ? (int)$data['id'] : false);
-
     $form_id = (isset($data['id']) ? (int)$data['id'] : 0);
     $name = (isset($data['name']) ? $data['name'] : WPFunctions::get()->__('New form', 'mailpoet'));
     $body = (isset($data['body']) ? $data['body'] : array());
@@ -211,56 +206,62 @@ class Forms extends APIEndpoint {
 
     if (!empty($errors)) {
       return $this->badRequest($errors);
-    } else {
-      return $this->successResponse(
-        Form::findOne($form->id)->asArray(),
-        array('is_widget' => $is_widget)
-      );
     }
+    $form = Form::findOne($form->id);
+    if(!$form instanceof Form) return $this->errorResponse();
+    return $this->successResponse(
+      $form->asArray(),
+      array('is_widget' => $is_widget)
+    );
   }
 
   function restore($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
-    if ($form === false) {
+    if ($form instanceof Form) {
+      $form->restore();
+      $form = Form::findOne($form->id);
+      if(!$form instanceof Form) return $this->errorResponse();
+      return $this->successResponse(
+        $form->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
       ));
-    } else {
-      $form->restore();
-      return $this->successResponse(
-        Form::findOne($form->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function trash($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
-    if ($form === false) {
+    if ($form instanceof Form) {
+      $form->trash();
+      $form = Form::findOne($form->id);
+      if(!$form instanceof Form) return $this->errorResponse();
+      return $this->successResponse(
+        $form->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
       ));
-    } else {
-      $form->trash();
-      return $this->successResponse(
-        Form::findOne($form->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
-    if ($form === false) {
+    if ($form instanceof Form) {
+      $form->delete();
+
+      return $this->successResponse(null, array('count' => 1));
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
       ));
-    } else {
-      $form->delete();
-      return $this->successResponse(null, array('count' => 1));
     }
   }
 
@@ -268,11 +269,7 @@ class Forms extends APIEndpoint {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $form = Form::findOne($id);
 
-    if ($form === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($form instanceof Form) {
       $data = array(
         'name' => sprintf(__('Copy of %s', 'mailpoet'), $form->name)
       );
@@ -282,11 +279,17 @@ class Forms extends APIEndpoint {
       if (!empty($errors)) {
         return $this->errorResponse($errors);
       } else {
+        $duplicate = Form::findOne($duplicate->id);
+        if(!$duplicate instanceof Form) return $this->errorResponse();
         return $this->successResponse(
-          Form::findOne($duplicate->id)->asArray(),
+          $duplicate->asArray(),
           array('count' => 1)
         );
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This form does not exist.', 'mailpoet')
+      ));
     }
   }
 
diff --git a/lib/API/JSON/v1/ImportExport.php b/lib/API/JSON/v1/ImportExport.php
index 40ee0aeb38..76a5ef16ab 100644
--- a/lib/API/JSON/v1/ImportExport.php
+++ b/lib/API/JSON/v1/ImportExport.php
@@ -45,9 +45,9 @@ class ImportExport extends APIEndpoint {
     if (!empty($errors)) {
       return $this->errorResponse($errors);
     } else {
-      return $this->successResponse(
-        Segment::findOne($segment->id)->asArray()
-      );
+      $segment = Segment::findOne($segment->id);
+      if(!$segment instanceof Segment) return $this->errorResponse();
+      return $this->successResponse($segment->asArray());
     }
   }
 
diff --git a/lib/API/JSON/v1/NewsletterTemplates.php b/lib/API/JSON/v1/NewsletterTemplates.php
index 1fd2176744..76afa7f806 100644
--- a/lib/API/JSON/v1/NewsletterTemplates.php
+++ b/lib/API/JSON/v1/NewsletterTemplates.php
@@ -18,14 +18,14 @@ class NewsletterTemplates extends APIEndpoint {
   function get($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $template = NewsletterTemplate::findOne($id);
-    if ($template === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This template does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($template instanceof NewsletterTemplate) {
       return $this->successResponse(
         $template->asArray()
       );
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This template does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -47,7 +47,7 @@ class NewsletterTemplates extends APIEndpoint {
     ignore_user_abort(true);
     if (!empty($data['newsletter_id'])) {
       $template = NewsletterTemplate::whereEqual('newsletter_id', $data['newsletter_id'])->findOne();
-      if (!empty($template)) {
+      if ($template instanceof NewsletterTemplate) {
         $data['id'] = $template->id;
       }
     }
@@ -60,8 +60,10 @@ class NewsletterTemplates extends APIEndpoint {
     if (!empty($errors)) {
       return $this->errorResponse($errors);
     } else {
+      $template = NewsletterTemplate::findOne($template->id);
+      if(!$template instanceof NewsletterTemplate) return $this->errorResponse();
       return $this->successResponse(
-        NewsletterTemplate::findOne($template->id)->asArray()
+        $template->asArray()
       );
     }
   }
@@ -69,13 +71,13 @@ class NewsletterTemplates extends APIEndpoint {
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $template = NewsletterTemplate::findOne($id);
-    if ($template === false) {
+    if ($template instanceof NewsletterTemplate) {
+      $template->delete();
+      return $this->successResponse(null, array('count' => 1));
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This template does not exist.', 'mailpoet')
       ));
-    } else {
-      $template->delete();
-      return $this->successResponse(null, array('count' => 1));
     }
   }
 }
diff --git a/lib/API/JSON/v1/Newsletters.php b/lib/API/JSON/v1/Newsletters.php
index c7fa71608e..5391f580e4 100644
--- a/lib/API/JSON/v1/Newsletters.php
+++ b/lib/API/JSON/v1/Newsletters.php
@@ -58,11 +58,7 @@ class Newsletters extends APIEndpoint {
   function get($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $newsletter = Newsletter::findOne($id);
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $newsletter = $newsletter
         ->withSegments()
         ->withOptions()
@@ -76,6 +72,10 @@ class Newsletters extends APIEndpoint {
 
       $newsletter = $this->wp->applyFilters('mailpoet_api_newsletters_get_after', $newsletter->asArray());
       return $this->successResponse($newsletter, ['preview_url' => $preview_url]);
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -96,7 +96,7 @@ class Newsletters extends APIEndpoint {
 
     if (!empty($data['template_id'])) {
       $template = NewsletterTemplate::whereEqual('id', $data['template_id'])->findOne();
-      if (!empty($template)) {
+      if ($template instanceof NewsletterTemplate) {
         $template = $template->asArray();
         $data['body'] = $template['body'];
       }
@@ -110,6 +110,7 @@ class Newsletters extends APIEndpoint {
     // Re-fetch newsletter to sync changes made by DB
     // updated_at column use CURRENT_TIMESTAMP for update and this change is not updated automatically by ORM
     $newsletter = Newsletter::findOne($newsletter->id);
+    if(!$newsletter instanceof Newsletter) return $this->errorResponse();
 
     if (!empty($segments)) {
       NewsletterSegment::where('newsletter_id', $newsletter->id)
@@ -146,6 +147,7 @@ class Newsletters extends APIEndpoint {
       }
       // reload newsletter with updated options
       $newsletter = Newsletter::filter('filterWithOptions', $newsletter->type)->findOne($newsletter->id);
+      if(!$newsletter instanceof Newsletter) return $this->errorResponse();
       // if this is a post notification, process newsletter options and update its schedule
       if ($newsletter->type === Newsletter::TYPE_NOTIFICATION) {
         // generate the new schedule from options and get the new "next run" date
@@ -226,54 +228,62 @@ class Newsletters extends APIEndpoint {
       Scheduler::createPostNotificationSendingTask($newsletter);
     }
 
-
+    $newsletter = Newsletter::findOne($newsletter->id);
+    if(!$newsletter instanceof Newsletter) return $this->errorResponse();
     return $this->successResponse(
-      Newsletter::findOne($newsletter->id)->asArray()
+      $newsletter->asArray()
     );
   }
 
   function restore($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $newsletter = Newsletter::findOne($id);
-    if ($newsletter === false) {
+    if ($newsletter instanceof Newsletter) {
+      $newsletter->restore();
+
+      $newsletter = Newsletter::findOne($newsletter->id);
+      if(!$newsletter instanceof Newsletter) return $this->errorResponse();
+
+      return $this->successResponse(
+        $newsletter->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
       ));
-    } else {
-      $newsletter->restore();
-      return $this->successResponse(
-        Newsletter::findOne($newsletter->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function trash($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $newsletter = Newsletter::findOne($id);
-    if ($newsletter === false) {
+    if ($newsletter instanceof Newsletter) {
+      $newsletter->trash();
+
+      $newsletter = Newsletter::findOne($newsletter->id);
+      if(!$newsletter instanceof Newsletter) return $this->errorResponse();
+      return $this->successResponse(
+        $newsletter->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
       ));
-    } else {
-      $newsletter->trash();
-      return $this->successResponse(
-        Newsletter::findOne($newsletter->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $newsletter = Newsletter::findOne($id);
-    if ($newsletter === false) {
+    if ($newsletter instanceof Newsletter) {
+      $newsletter->delete();
+      return $this->successResponse(null, array('count' => 1));
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
       ));
-    } else {
-      $newsletter->delete();
-      return $this->successResponse(null, array('count' => 1));
     }
   }
 
@@ -281,11 +291,7 @@ class Newsletters extends APIEndpoint {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $newsletter = Newsletter::findOne($id);
 
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $data = array(
         'subject' => sprintf(__('Copy of %s', 'mailpoet'), $newsletter->subject)
       );
@@ -296,11 +302,17 @@ class Newsletters extends APIEndpoint {
         return $this->errorResponse($errors);
       } else {
         $this->wp->doAction('mailpoet_api_newsletters_duplicate_after', $newsletter, $duplicate);
+        $duplicate = Newsletter::findOne($duplicate->id);
+        if(!$duplicate instanceof Newsletter) return $this->errorResponse();
         return $this->successResponse(
-          Newsletter::findOne($duplicate->id)->asArray(),
+          $duplicate->asArray(),
           array('count' => 1)
         );
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -314,11 +326,7 @@ class Newsletters extends APIEndpoint {
     $id = (isset($data['id'])) ? (int)$data['id'] : false;
     $newsletter = Newsletter::findOne($id);
 
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $newsletter->body = $data['body'];
       $newsletter->save();
       $subscriber = Subscriber::getCurrentWPUser();
@@ -330,10 +338,16 @@ class Newsletters extends APIEndpoint {
       // strip protocol to avoid mix content error
       $preview_url = preg_replace('{^https?:}i', '', $preview_url);
 
+      $newsletter = Newsletter::findOne($newsletter->id);
+      if(!$newsletter instanceof Newsletter) return $this->errorResponse();
       return $this->successResponse(
-        Newsletter::findOne($newsletter->id)->asArray(),
+        $newsletter->asArray(),
         array('preview_url' => $preview_url)
       );
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -347,11 +361,7 @@ class Newsletters extends APIEndpoint {
     $id = (isset($data['id'])) ? (int)$data['id'] : false;
     $newsletter = Newsletter::findOne($id);
 
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $renderer = new Renderer($newsletter, $preview = true);
       $rendered_newsletter = $renderer->render();
       $divider = '***MailPoet***';
@@ -376,7 +386,7 @@ class Newsletters extends APIEndpoint {
         $rendered_newsletter['subject'],
         $rendered_newsletter['body']['html'],
         $rendered_newsletter['body']['text']
-      ) = explode($divider, $shortcodes->replace($body));
+        ) = explode($divider, $shortcodes->replace($body));
       $rendered_newsletter['id'] = $newsletter->id;
 
       try {
@@ -386,7 +396,7 @@ class Newsletters extends APIEndpoint {
             $mailer = false,
             $sender = false,
             $reply_to = false
-        );
+          );
         $extra_params = array('unsubscribe_url' => WPFunctions::get()->homeUrl());
         $result = $mailer->send($rendered_newsletter, $data['subscriber'], $extra_params);
 
@@ -397,8 +407,11 @@ class Newsletters extends APIEndpoint {
           );
           return $this->errorResponse(array(APIError::BAD_REQUEST => $error));
         } else {
+          $newsletter = Newsletter::findOne($newsletter->id);
+          if(!$newsletter instanceof Newsletter) return $this->errorResponse();
+
           return $this->successResponse(
-            Newsletter::findOne($id)->asArray()
+            $newsletter->asArray()
           );
         }
       } catch (\Exception $e) {
@@ -406,6 +419,10 @@ class Newsletters extends APIEndpoint {
           $e->getCode() => $e->getMessage()
         ));
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -494,10 +511,10 @@ class Newsletters extends APIEndpoint {
       // try to load template data
       $template_id = (isset($data['template']) ? (int)$data['template'] : false);
       $template = NewsletterTemplate::findOne($template_id);
-      if ($template === false) {
-        $newsletter->body = array();
-      } else {
+      if ($template instanceof NewsletterTemplate) {
         $newsletter->body = $template->body;
+      } else {
+        $newsletter->body = array();
       }
     }
 
@@ -533,8 +550,10 @@ class Newsletters extends APIEndpoint {
         Scheduler::processPostNotificationSchedule($newsletter);
       }
 
+      $newsletter = Newsletter::findOne($newsletter->id);
+      if(!$newsletter instanceof Newsletter) return $this->errorResponse();
       return $this->successResponse(
-        Newsletter::findOne($newsletter->id)->asArray()
+        $newsletter->asArray()
       );
     }
   }
diff --git a/lib/API/JSON/v1/Segments.php b/lib/API/JSON/v1/Segments.php
index d4350cff84..4bbd63d481 100644
--- a/lib/API/JSON/v1/Segments.php
+++ b/lib/API/JSON/v1/Segments.php
@@ -40,12 +40,12 @@ class Segments extends APIEndpoint {
   function get($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $segment = Segment::findOne($id);
-    if ($segment === false) {
+    if ($segment instanceof Segment) {
+      return $this->successResponse($segment->asArray());
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
       ));
-    } else {
-      return $this->successResponse($segment->asArray());
     }
   }
 
@@ -77,8 +77,10 @@ class Segments extends APIEndpoint {
     if (!empty($errors)) {
       return $this->badRequest($errors);
     } else {
+      $segment = Segment::findOne($segment->id);
+      if(!$segment instanceof Segment) return $this->errorResponse();
       return $this->successResponse(
-        Segment::findOne($segment->id)->asArray()
+        $segment->asArray()
       );
     }
   }
@@ -86,45 +88,49 @@ class Segments extends APIEndpoint {
   function restore($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $segment = Segment::findOne($id);
-    if ($segment === false) {
+    if ($segment instanceof Segment) {
+      $segment->restore();
+      $segment = Segment::findOne($segment->id);
+      if(!$segment instanceof Segment) return $this->errorResponse();
+      return $this->successResponse(
+        $segment->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
       ));
-    } else {
-      $segment->restore();
-      return $this->successResponse(
-        Segment::findOne($segment->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function trash($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $segment = Segment::findOne($id);
-    if ($segment === false) {
+    if ($segment instanceof Segment) {
+      $segment->trash();
+      $segment = Segment::findOne($segment->id);
+      if(!$segment instanceof Segment) return $this->errorResponse();
+      return $this->successResponse(
+        $segment->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
       ));
-    } else {
-      $segment->trash();
-      return $this->successResponse(
-        Segment::findOne($segment->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $segment = Segment::findOne($id);
-    if ($segment === false) {
+    if ($segment instanceof Segment) {
+      $segment->delete();
+      return $this->successResponse(null, array('count' => 1));
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
       ));
-    } else {
-      $segment->delete();
-      return $this->successResponse(null, array('count' => 1));
     }
   }
 
@@ -132,11 +138,7 @@ class Segments extends APIEndpoint {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $segment = Segment::findOne($id);
 
-    if ($segment === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($segment instanceof Segment) {
       $data = array(
         'name' => sprintf(__('Copy of %s', 'mailpoet'), $segment->name)
       );
@@ -146,11 +148,17 @@ class Segments extends APIEndpoint {
       if (!empty($errors)) {
         return $this->errorResponse($errors);
       } else {
+        $duplicate = Segment::findOne($duplicate->id);
+        if(!$duplicate instanceof Segment) return $this->errorResponse();
         return $this->successResponse(
-          Segment::findOne($duplicate->id)->asArray(),
+          $duplicate->asArray(),
           array('count' => 1)
         );
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This list does not exist.', 'mailpoet')
+      ));
     }
   }
 
diff --git a/lib/API/JSON/v1/SendingQueue.php b/lib/API/JSON/v1/SendingQueue.php
index 25660e6753..83e49a7448 100644
--- a/lib/API/JSON/v1/SendingQueue.php
+++ b/lib/API/JSON/v1/SendingQueue.php
@@ -28,7 +28,7 @@ class SendingQueue extends APIEndpoint {
     // check that the newsletter exists
     $newsletter = Newsletter::findOneWithOptions($newsletter_id);
 
-    if ($newsletter === false) {
+    if (!$newsletter instanceof Newsletter) {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
       ));
@@ -59,7 +59,7 @@ class SendingQueue extends APIEndpoint {
       ->where('queues.newsletter_id', $newsletter->id)
       ->where('tasks.status', SendingQueueModel::STATUS_SCHEDULED)
       ->findOne();
-    if ($scheduled_queue) {
+    if ($scheduled_queue instanceof SendingQueueModel) {
       $queue = SendingTask::createFromQueue($scheduled_queue);
     } else {
       $queue = SendingTask::create();
@@ -108,11 +108,7 @@ class SendingQueue extends APIEndpoint {
     );
     $newsletter = Newsletter::findOne($newsletter_id);
 
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $queue = $newsletter->getQueue();
 
       if ($queue === false) {
@@ -125,6 +121,10 @@ class SendingQueue extends APIEndpoint {
           $newsletter->getQueue()->asArray()
         );
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 
@@ -134,11 +134,7 @@ class SendingQueue extends APIEndpoint {
       : false
     );
     $newsletter = Newsletter::findOne($newsletter_id);
-    if ($newsletter === false) {
-      return $this->errorResponse(array(
-        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
-      ));
-    } else {
+    if ($newsletter instanceof Newsletter) {
       $queue = $newsletter->getQueue();
 
       if ($queue === false) {
@@ -151,6 +147,10 @@ class SendingQueue extends APIEndpoint {
           $newsletter->getQueue()->asArray()
         );
       }
+    } else {
+      return $this->errorResponse(array(
+        APIError::NOT_FOUND => WPFunctions::get()->__('This newsletter does not exist.', 'mailpoet')
+      ));
     }
   }
 }
diff --git a/lib/API/JSON/v1/Subscribers.php b/lib/API/JSON/v1/Subscribers.php
index ce2fc71a67..88dad9282f 100644
--- a/lib/API/JSON/v1/Subscribers.php
+++ b/lib/API/JSON/v1/Subscribers.php
@@ -120,7 +120,7 @@ class Subscribers extends APIEndpoint {
 
     $recaptcha = $this->settings->get('re_captcha');
 
-    if (!$form) {
+    if (!$form instanceof Form) {
       return $this->badRequest(array(
         APIError::BAD_REQUEST => WPFunctions::get()->__('Please specify a valid form ID.', 'mailpoet')
       ));
@@ -266,45 +266,49 @@ class Subscribers extends APIEndpoint {
   function restore($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $subscriber = Subscriber::findOne($id);
-    if ($subscriber === false) {
+    if ($subscriber instanceof Subscriber) {
+      $subscriber->restore();
+      $subscriber = Subscriber::findOne($subscriber->id);
+      if(!$subscriber instanceof Subscriber) return $this->errorResponse();
+      return $this->successResponse(
+        $subscriber->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This subscriber does not exist.', 'mailpoet')
       ));
-    } else {
-      $subscriber->restore();
-      return $this->successResponse(
-        Subscriber::findOne($subscriber->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function trash($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $subscriber = Subscriber::findOne($id);
-    if ($subscriber === false) {
+    if ($subscriber instanceof Subscriber) {
+      $subscriber->trash();
+      $subscriber = Subscriber::findOne($subscriber->id);
+      if(!$subscriber instanceof Subscriber) return $this->errorResponse();
+      return $this->successResponse(
+        $subscriber->asArray(),
+        array('count' => 1)
+      );
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This subscriber does not exist.', 'mailpoet')
       ));
-    } else {
-      $subscriber->trash();
-      return $this->successResponse(
-        Subscriber::findOne($subscriber->id)->asArray(),
-        array('count' => 1)
-      );
     }
   }
 
   function delete($data = array()) {
     $id = (isset($data['id']) ? (int)$data['id'] : false);
     $subscriber = Subscriber::findOne($id);
-    if ($subscriber === false) {
+    if ($subscriber instanceof Subscriber) {
+      $subscriber->delete();
+      return $this->successResponse(null, array('count' => 1));
+    } else {
       return $this->errorResponse(array(
         APIError::NOT_FOUND => WPFunctions::get()->__('This subscriber does not exist.', 'mailpoet')
       ));
-    } else {
-      $subscriber->delete();
-      return $this->successResponse(null, array('count' => 1));
     }
   }
 
diff --git a/lib/API/MP/v1/API.php b/lib/API/MP/v1/API.php
index ed231ddb88..dec16a37b4 100644
--- a/lib/API/MP/v1/API.php
+++ b/lib/API/MP/v1/API.php
@@ -268,6 +268,9 @@ class API {
 
     // reload list to get the saved created|updated|delete dates/other fields
     $new_list = Segment::findOne($new_list->id);
+    if (!$new_list instanceof Segment) {
+      throw new \Exception(WPFunctions::get()->__('Failed to add list', 'mailpoet'));
+    }
 
     return $new_list->asArray();
   }
diff --git a/lib/Config/Capabilities.php b/lib/Config/Capabilities.php
index 7dde2d9623..3fd047bc65 100644
--- a/lib/Config/Capabilities.php
+++ b/lib/Config/Capabilities.php
@@ -2,6 +2,7 @@
 
 namespace MailPoet\Config;
 use MailPoet\WP\Functions as WPFunctions;
+use WP_Role;
 
 class Capabilities {
   const MEMBERS_CAP_GROUP_NAME = 'mailpoet';
@@ -35,7 +36,7 @@ class Capabilities {
         if (!isset($role_objects[$role])) {
           $role_objects[$role] = WPFunctions::get()->getRole($role);
         }
-        if (!is_object($role_objects[$role])) continue;
+        if (!$role_objects[$role] instanceof WP_Role) continue;
         $role_objects[$role]->add_cap($name);
       }
     }
@@ -49,7 +50,7 @@ class Capabilities {
         if (!isset($role_objects[$role])) {
           $role_objects[$role] = WPFunctions::get()->getRole($role);
         }
-        if (!is_object($role_objects[$role])) continue;
+        if (!$role_objects[$role] instanceof WP_Role) continue;
         $role_objects[$role]->remove_cap($name);
       }
     }
diff --git a/lib/Config/Menu.php b/lib/Config/Menu.php
index d55a004184..4bd4381c65 100644
--- a/lib/Config/Menu.php
+++ b/lib/Config/Menu.php
@@ -748,7 +748,7 @@ class Menu {
   function formEditor() {
     $id = (isset($_GET['id']) ? (int)$_GET['id'] : 0);
     $form = Form::findOne($id);
-    if ($form !== false) {
+    if ($form instanceof Form) {
       $form = $form->asArray();
     }
 
diff --git a/lib/Config/Renderer.php b/lib/Config/Renderer.php
index 6907147775..a7f2f47c29 100644
--- a/lib/Config/Renderer.php
+++ b/lib/Config/Renderer.php
@@ -121,9 +121,13 @@ class Renderer {
   }
 
   function getAssetManifest($manifest_file) {
-    return (is_readable($manifest_file)) ?
-      json_decode(file_get_contents($manifest_file), true) :
-      false;
+    if (is_readable($manifest_file)) {
+      $contents = file_get_contents($manifest_file);
+      if (is_string($contents)) {
+        return json_decode($contents, true);
+      }
+    }
+    return false;
   }
 
   function getJsAsset($asset) {
diff --git a/lib/Config/RequirementsChecker.php b/lib/Config/RequirementsChecker.php
index 80908c58d6..69cd1e621a 100644
--- a/lib/Config/RequirementsChecker.php
+++ b/lib/Config/RequirementsChecker.php
@@ -47,7 +47,10 @@ class RequirementsChecker {
     );
     $results = array();
     foreach ($available_tests as $test) {
-      $results[$test] = call_user_func(array($this, 'check' .  $test));
+      $callback = [$this, 'check' . $test];
+      if (is_callable($callback)) {
+        $results[$test] = call_user_func($callback);
+      }
     }
     return $results;
   }
diff --git a/lib/Config/Updater.php b/lib/Config/Updater.php
index ccb3b71296..0444830492 100644
--- a/lib/Config/Updater.php
+++ b/lib/Config/Updater.php
@@ -28,9 +28,9 @@ class Updater {
   function init() {
     WPFunctions::get()->addFilter('pre_set_site_transient_update_plugins', array($this, 'checkForUpdate'));
   }
-  
+
   function checkForUpdate($update_transient) {
-    if (!is_object($update_transient)) {
+    if (!$update_transient instanceof \stdClass) {
       $update_transient = new \stdClass;
     }
 
diff --git a/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php b/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
index f80c6de797..251926a326 100644
--- a/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
+++ b/lib/Cron/Workers/SendingQueue/Tasks/Newsletter.php
@@ -121,7 +121,9 @@ class Newsletter {
     if (!$queue_errors) {
       // verify that the rendered body was successfully saved
       $queue = SendingQueueModel::findOne($queue->id);
-      $queue_errors = ($queue->validate() !== true);
+      if ($queue instanceof SendingQueueModel) {
+        $queue_errors = ($queue->validate() !== true);
+      }
     }
     if ($queue_errors) {
       $this->stopNewsletterPreProcessing(sprintf('QUEUE-%d-SAVE', $queue->id));
diff --git a/lib/Cron/Workers/StatsNotifications/Worker.php b/lib/Cron/Workers/StatsNotifications/Worker.php
index 2efe00e48c..3ebab65895 100644
--- a/lib/Cron/Workers/StatsNotifications/Worker.php
+++ b/lib/Cron/Workers/StatsNotifications/Worker.php
@@ -9,6 +9,7 @@ use MailPoet\Mailer\Mailer;
 use MailPoet\Models\Newsletter;
 use MailPoet\Models\NewsletterLink;
 use MailPoet\Models\ScheduledTask;
+use MailPoet\Models\StatsNotification;
 use MailPoet\Settings\SettingsController;
 use MailPoet\Tasks\Sending;
 use MailPoet\WP\Functions as WPFunctions;
@@ -81,8 +82,11 @@ class Worker {
 
   private function getNewsletter(ScheduledTask $task) {
     $statsNotificationModel = $task->statsNotification()->findOne();
+    if (!$statsNotificationModel instanceof StatsNotification) {
+      throw new \Exception('Newsletter not found');
+    }
     $newsletter = $statsNotificationModel->newsletter()->findOne();
-    if (!$newsletter) {
+    if (!$newsletter instanceof Newsletter) {
       throw new \Exception('Newsletter not found');
     }
     return $newsletter
diff --git a/lib/Listing/BulkActionController.php b/lib/Listing/BulkActionController.php
index 6b7166d6d4..1875b9026b 100644
--- a/lib/Listing/BulkActionController.php
+++ b/lib/Listing/BulkActionController.php
@@ -20,10 +20,13 @@ class BulkActionController {
     unset($data['action']);
 
     $action_class = $this->factory->getActionClass($model_class, $bulk_action_method);
+    $callback = [$action_class, $bulk_action_method];
 
-    return call_user_func_array(
-      array($action_class, $bulk_action_method),
-      array($this->handler->getSelection($model_class, $data['listing']), $data)
-    );
+    if (is_callable($callback)) {
+      return call_user_func_array(
+        $callback,
+        array($this->handler->getSelection($model_class, $data['listing']), $data)
+      );
+    }
   }
 }
diff --git a/lib/Models/Model.php b/lib/Models/Model.php
index c7c7c8e2e8..cc5a588b84 100644
--- a/lib/Models/Model.php
+++ b/lib/Models/Model.php
@@ -22,8 +22,8 @@ if (!defined('ABSPATH')) exit;
  * @method $this useIdColumn($id_column)
  * @method $this|bool findOne($id=null)
  * @method static static|bool findOne($id=null)
- * @method array|\IdiormResultSet findMany()
- * @method static array|\IdiormResultSet findMany()
+ * @method array findMany()
+ * @method static array findMany()
  * @method \IdiormResultSet findResultSet()
  * @method array findArray()
  * @method static array findArray()
@@ -162,7 +162,7 @@ class Model extends \Sudzy\ValidModel {
     }
 
     if ($model === false) {
-      if (!empty($onCreate)) {
+      if (!empty($onCreate) && is_callable($onCreate)) {
         $data = $onCreate($data);
       }
       $model = static::create();
diff --git a/lib/Models/ModelValidator.php b/lib/Models/ModelValidator.php
index 40c3583e26..bc0f6fcf51 100644
--- a/lib/Models/ModelValidator.php
+++ b/lib/Models/ModelValidator.php
@@ -26,7 +26,10 @@ class ModelValidator extends \Sudzy\Engine {
     $_this = $this;
     foreach ($this->validators as $validator => $action) {
       $this->addValidator($validator, function($params) use ($action, $_this) {
-        return call_user_func(array($_this, $action), $params);
+        $callback = [$_this, $action];
+        if (is_callable($callback)) {
+          return call_user_func($callback, $params);
+        }
       });
     }
   }
diff --git a/lib/Models/Newsletter.php b/lib/Models/Newsletter.php
index a7084df92e..04c3a8481b 100644
--- a/lib/Models/Newsletter.php
+++ b/lib/Models/Newsletter.php
@@ -29,6 +29,8 @@ if (!defined('ABSPATH')) exit;
  * @property string $subject
  * @property string $body
  * @property string|null $schedule
+ * @property boolean|null $isScheduled
+ * @property string|null $scheduledAt
  */
 
 class Newsletter extends Model {
diff --git a/lib/Models/Subscriber.php b/lib/Models/Subscriber.php
index ff2aa8fc02..2d7269809f 100644
--- a/lib/Models/Subscriber.php
+++ b/lib/Models/Subscriber.php
@@ -218,7 +218,7 @@ class Subscriber extends Model {
           return self::filter('withoutSegments');
         } else {
           $segment = Segment::findOne($value);
-          if ($segment !== false) {
+          if ($segment instanceof Segment) {
             return $segment->subscribers();
           }
         }
@@ -459,10 +459,10 @@ class Subscriber extends Model {
       ->where('subscriber_id', $this->id())
       ->findOne();
 
-    if ($custom_field === false) {
-      return $default;
-    } else {
+    if ($custom_field instanceof SubscriberCustomField) {
       return $custom_field->value;
+    } else {
+      return $default;
     }
   }
 
@@ -495,7 +495,10 @@ class Subscriber extends Model {
 
   function setUnconfirmedData(array $subscriber_data) {
     $subscriber_data = self::filterOutReservedColumns($subscriber_data);
-    $this->unconfirmed_data = json_encode($subscriber_data);
+    $encoded = json_encode($subscriber_data);
+    if (is_string($encoded)) {
+      $this->unconfirmed_data = $encoded;
+    }
   }
 
   function getUnconfirmedData() {
@@ -511,7 +514,7 @@ class Subscriber extends Model {
     $segment_id = (isset($data['segment_id']) ? (int)$data['segment_id'] : 0);
     $segment = Segment::findOne($segment_id);
 
-    if ($segment === false) return false;
+    if (!$segment instanceof Segment) return false;
 
     $count = parent::bulkAction($orm,
       function($subscriber_ids) use($segment) {
@@ -531,7 +534,7 @@ class Subscriber extends Model {
     $segment_id = (isset($data['segment_id']) ? (int)$data['segment_id'] : 0);
     $segment = Segment::findOne($segment_id);
 
-    if ($segment === false) return false;
+    if (!$segment instanceof Segment) return false;
 
     $count = parent::bulkAction($orm,
       function($subscriber_ids) use($segment) {
@@ -552,7 +555,7 @@ class Subscriber extends Model {
     $segment_id = (isset($data['segment_id']) ? (int)$data['segment_id'] : 0);
     $segment = Segment::findOne($segment_id);
 
-    if ($segment === false) return false;
+    if (!$segment instanceof Segment) return false;
 
     $count = $orm->count();
 
diff --git a/lib/Models/SubscriberCustomField.php b/lib/Models/SubscriberCustomField.php
index 25cd5dfb00..991fab6432 100644
--- a/lib/Models/SubscriberCustomField.php
+++ b/lib/Models/SubscriberCustomField.php
@@ -15,10 +15,10 @@ class SubscriberCustomField extends Model {
 
   static function createOrUpdate($data = array()) {
     $custom_field = CustomField::findOne($data['custom_field_id']);
-    if ($custom_field === false) {
-      return false;
-    } else {
+    if ($custom_field instanceof CustomField) {
       $custom_field = $custom_field->asArray();
+    } else {
+      return false;
     }
 
     if ($custom_field['type'] === 'date') {
diff --git a/lib/Newsletter/Links/Links.php b/lib/Newsletter/Links/Links.php
index 17879ad3df..7922ad8ab3 100644
--- a/lib/Newsletter/Links/Links.php
+++ b/lib/Newsletter/Links/Links.php
@@ -179,7 +179,7 @@ class Links {
         ->findOne();
       // convert either only link shortcodes or all hashes links if "convert all"
       // option is specified
-      if ($newsletter_link &&
+      if (($newsletter_link instanceof NewsletterLink) &&
         (preg_match('/\[link:/', $newsletter_link->url) || $convert_all)
       ) {
         $content = str_replace($link, $newsletter_link->url, $content);
diff --git a/lib/Router/Router.php b/lib/Router/Router.php
index 2938ec776b..ede6e355ac 100644
--- a/lib/Router/Router.php
+++ b/lib/Router/Router.php
@@ -55,13 +55,13 @@ class Router {
       return $this->terminateRequest(self::RESPONE_FORBIDDEN, WPFunctions::get()->__('You do not have the required permissions.', 'mailpoet'));
     }
     WPFunctions::get()->doAction('mailpoet_conflict_resolver_router_url_query_parameters');
-    return call_user_func(
-      [
-        $endpoint,
-        $this->endpoint_action,
-      ],
-      $this->data
-    );
+    $callback = [
+      $endpoint,
+      $this->endpoint_action,
+    ];
+    if (is_callable($callback)) {
+      return call_user_func($callback, $this->data);
+    }
   }
 
   static function decodeRequestData($data) {
diff --git a/lib/Segments/BulkAction.php b/lib/Segments/BulkAction.php
index 9a47f69fb0..7e78912c5c 100644
--- a/lib/Segments/BulkAction.php
+++ b/lib/Segments/BulkAction.php
@@ -26,14 +26,14 @@ class BulkAction {
       throw new \InvalidArgumentException('Missing segment id');
     }
     $segment = Segment::findOne($this->data['listing']['filter']['segment']);
-    if ($segment) {
+    if ($segment instanceof Segment) {
       $segment = $segment->asArray();
     }
     return $this->applySegment($segment);
   }
 
   /**
-   * @param array $segment
+   * @param array|bool $segment
    *
    * @return array
    * @throws \Exception
diff --git a/lib/Segments/SubscribersListings.php b/lib/Segments/SubscribersListings.php
index ad8daae211..50eea133b5 100644
--- a/lib/Segments/SubscribersListings.php
+++ b/lib/Segments/SubscribersListings.php
@@ -23,7 +23,7 @@ class SubscribersListings {
       throw new \InvalidArgumentException('Missing segment id');
     }
     $segment = Segment::findOne($data['filter']['segment']);
-    return $this->getListings($data, $segment ?: null);
+    return $this->getListings($data, $segment instanceof Segment ? $segment : null);
 
   }
 
diff --git a/lib/Settings/SettingsController.php b/lib/Settings/SettingsController.php
index d2704d1741..4ed4701b47 100644
--- a/lib/Settings/SettingsController.php
+++ b/lib/Settings/SettingsController.php
@@ -129,7 +129,7 @@ class SettingsController {
 
   private function fetchValue($key) {
     $setting = Setting::where('name', $key)->findOne();
-    if ($setting === false) {
+    if (!$setting instanceof Setting) {
       return null;
     }
     if (is_serialized($setting->value)) {
diff --git a/lib/Subscribers/ImportExport/Export/Export.php b/lib/Subscribers/ImportExport/Export/Export.php
index 976b778102..e32a3c0d9d 100644
--- a/lib/Subscribers/ImportExport/Export/Export.php
+++ b/lib/Subscribers/ImportExport/Export/Export.php
@@ -59,6 +59,7 @@ class Export {
   }
 
   function process() {
+    $processed_subscribers = 0;
     $this->default_subscribers_getter->reset();
     try {
       if (is_writable($this->export_path) === false) {
@@ -67,12 +68,13 @@ class Export {
       if (!extension_loaded('zip')) {
         throw new \Exception(__('Export requires a ZIP extension to be installed on the host.', 'mailpoet'));
       }
-      $processed_subscribers = call_user_func(
-        array(
-          $this,
-          'generate' . strtoupper($this->export_format_option)
-        )
-      );
+      $callback = [
+        $this,
+        'generate' . strtoupper($this->export_format_option)
+      ];
+      if (is_callable($callback)) {
+        $processed_subscribers = call_user_func($callback);
+      }
     } catch (\Exception $e) {
       throw new \Exception($e->getMessage());
     }
diff --git a/lib/Tasks/Sending.php b/lib/Tasks/Sending.php
index 4510f8cfda..a1f6864eac 100644
--- a/lib/Tasks/Sending.php
+++ b/lib/Tasks/Sending.php
@@ -235,7 +235,10 @@ class Sending {
 
   public function __call($name, $args) {
     $obj = method_exists($this->queue, $name) ? $this->queue : $this->task;
-    return call_user_func_array(array($obj, $name), $args);
+    $callback = [$obj, $name];
+    if (is_callable($callback)) {
+      return call_user_func_array($callback, $args);
+    }
   }
 
   private function isQueueProperty($prop) {
diff --git a/lib/Tasks/State.php b/lib/Tasks/State.php
index 89fda969a4..8bd34fc081 100644
--- a/lib/Tasks/State.php
+++ b/lib/Tasks/State.php
@@ -73,7 +73,7 @@ class State
     $queue = $newsletter = null;
     if ($task->type === Sending::TASK_TYPE) {
       $queue = SendingQueue::where('task_id', $task->id)->findOne();
-      $newsletter = $queue ? $queue->newsletter()->findOne() : null;
+      $newsletter = $queue instanceof SendingQueue ? $queue->newsletter()->findOne() : null;
     }
     return [
       'id' => (int)$task->id,
diff --git a/lib/Twig/I18n.php b/lib/Twig/I18n.php
index ee19f75a47..e2344ef0ee 100644
--- a/lib/Twig/I18n.php
+++ b/lib/Twig/I18n.php
@@ -92,7 +92,7 @@ class I18n extends AbstractExtension {
       $date = strtotime($date);
     }
 
-    return WPFunctions::get()->getDateFromGmt(date('Y-m-d H:i:s', $date), $date_format);
+    return WPFunctions::get()->getDateFromGmt(date('Y-m-d H:i:s', (int)$date), $date_format);
   }
 
   private function setTextDomain($args = array()) {
diff --git a/lib/Util/ProgressBar.php b/lib/Util/ProgressBar.php
index a86f6552c0..b5e24d163e 100644
--- a/lib/Util/ProgressBar.php
+++ b/lib/Util/ProgressBar.php
@@ -51,12 +51,14 @@ if (!class_exists('ProgressBar', false)) {
      * @return array|false Array of counters
      */
     private function readProgress() {
-      if (file_exists($this->filename)) {
-        $json_content = file_get_contents($this->filename);
-        return json_decode($json_content);
-      } else {
+      if (!file_exists($this->filename)) {
         return false;
       }
+      $json_content = file_get_contents($this->filename);
+      if (is_string($json_content)) {
+        return json_decode($json_content);
+      }
+      return false;
     }
 
     /**
diff --git a/lib/Util/Sudzy/Engine.php b/lib/Util/Sudzy/Engine.php
index 2ba666faec..f330a64aeb 100644
--- a/lib/Util/Sudzy/Engine.php
+++ b/lib/Util/Sudzy/Engine.php
@@ -37,7 +37,10 @@ class Engine
 
     public function executeOne($check, $val, $params=array())
     {
-        return call_user_func(__NAMESPACE__ .'\Engine::'.$check, $val, $params);
+      $callback = __NAMESPACE__ .'\Engine::'.$check;
+      if (is_callable($callback)) {
+        return call_user_func($callback, $val, $params);
+      }
     }
 
     /**
diff --git a/lib/Util/XLSXWriter.php b/lib/Util/XLSXWriter.php
index 1f37ff9592..b1964e0ae2 100644
--- a/lib/Util/XLSXWriter.php
+++ b/lib/Util/XLSXWriter.php
@@ -201,7 +201,7 @@ class XLSXWriter
     $max_cell_tag = '<dimension ref="A1:' . $max_cell . '"/>';
     $padding_length = $sheet->max_cell_tag_end - $sheet->max_cell_tag_start - strlen($max_cell_tag);
     $sheet->file_writer->fseek($sheet->max_cell_tag_start);
-    $sheet->file_writer->write($max_cell_tag.str_repeat(" ", $padding_length));
+    $sheet->file_writer->write($max_cell_tag . str_repeat(" ", (int)$padding_length));
     $sheet->file_writer->close();
     $sheet->finalized=true;
   }
diff --git a/lib/WP/Functions.php b/lib/WP/Functions.php
index fe0c481bb1..a830725571 100644
--- a/lib/WP/Functions.php
+++ b/lib/WP/Functions.php
@@ -1,6 +1,8 @@
 <?php
 namespace MailPoet\WP;
 
+use WP_Error;
+
 class Functions {
 
   static private $instance;
@@ -240,14 +242,16 @@ class Functions {
   }
 
   /**
-   * @param  string|array $deprecated
+   * @param string|array $args
+   * @param string|array $deprecated
+   * @return array|int|WP_Error
    */
   function getTerms($args = array(), $deprecated = '') {
     return get_terms($args, $deprecated);
   }
 
   /**
-   * @param  int|boolean $user_id
+   * @param int|boolean $user_id
    */
   function getTheAuthorMeta($field = '', $user_id = false) {
     return get_the_author_meta($field, $user_id);