diff --git a/lib-3rd-party/pquery/gan_node_html.php b/lib-3rd-party/pquery/gan_node_html.php
index 91523caa71..ea5030d5a4 100644
--- a/lib-3rd-party/pquery/gan_node_html.php
+++ b/lib-3rd-party/pquery/gan_node_html.php
@@ -1453,7 +1453,7 @@ class DomNode implements IQuery {
 	 * @return bool
 	 */
 	function hasClass($className) {
-		return ($className && preg_match('`\b'.preg_quote($className).'\b`si', $this->class));
+		return ($className && preg_match('`\b'.preg_quote($className).'\b`si', (string)$this->class));
 	}
 
 	/**
@@ -1466,11 +1466,11 @@ class DomNode implements IQuery {
 		}
 		$class = $this->class;
 		foreach ($className as $c) {
-			if (!(preg_match('`\b'.preg_quote($c).'\b`si', $class) > 0)) {
+			if (!(preg_match('`\b'.preg_quote($c).'\b`si', (string)$class) > 0)) {
 				$class .= ' '.$c;
 			}
 		}
-		 $this->class = trim($class);
+		 $this->class = trim((string)$class);
 	}
 
 	/**
diff --git a/lib/Mailer/Methods/ErrorMappers/SMTPMapper.php b/lib/Mailer/Methods/ErrorMappers/SMTPMapper.php
index 117d87a040..f6228a3ba3 100644
--- a/lib/Mailer/Methods/ErrorMappers/SMTPMapper.php
+++ b/lib/Mailer/Methods/ErrorMappers/SMTPMapper.php
@@ -39,7 +39,7 @@ class SMTPMapper {
 
   public function getErrorFromLog($log, $subscriber) {
     // extract error message from log
-    preg_match('/!! (.*?)>>/ism', $log, $message);
+    preg_match('/!! (.*?)>>/ism', (string)$log, $message);
     if (!empty($message[1])) {
       $message = $message[1];
       // remove line breaks from the message due to how logger's dump() method works
diff --git a/lib/Newsletter/Renderer/Blocks/Text.php b/lib/Newsletter/Renderer/Blocks/Text.php
index a197cb40f4..4caa29cfd8 100644
--- a/lib/Newsletter/Renderer/Blocks/Text.php
+++ b/lib/Newsletter/Renderer/Blocks/Text.php
@@ -101,7 +101,7 @@ class Text {
         $paragraph->remove();
         continue;
       }
-      $style = $paragraph->style;
+      $style = (string)$paragraph->style;
       if (!preg_match('/text-align/i', $style)) {
         $style = 'text-align: left;' . $style;
       }
diff --git a/lib/Newsletter/Renderer/StylesHelper.php b/lib/Newsletter/Renderer/StylesHelper.php
index 7c374f2fc7..ff06043373 100644
--- a/lib/Newsletter/Renderer/StylesHelper.php
+++ b/lib/Newsletter/Renderer/StylesHelper.php
@@ -104,13 +104,13 @@ class StylesHelper {
       $textAlignment = isset($block['styles']['block']['textAlign']) ?
         strtolower($block['styles']['block']['textAlign']) :
         '';
-      if (preg_match('/center|right|justify/i', $textAlignment)) {
+      if (preg_match('/center|right|justify/i', (string)$textAlignment)) {
         return $block;
       }
       $block['styles']['block']['textAlign'] = 'left';
       return $block;
     }
-    return (preg_match('/text-align.*?[center|justify|right]/i', $block)) ?
+    return (preg_match('/text-align.*?[center|justify|right]/i', (string)$block)) ?
       $block :
       $block . 'text-align:left;';
   }
diff --git a/lib/Subscribers/ImportExport/Export/Export.php b/lib/Subscribers/ImportExport/Export/Export.php
index e53a6d2fb9..1dfcde38a9 100644
--- a/lib/Subscribers/ImportExport/Export/Export.php
+++ b/lib/Subscribers/ImportExport/Export/Export.php
@@ -113,7 +113,7 @@ class Export {
       throw new \Exception(__('Failed opening file for export.', 'mailpoet'));
     }
     $formatCSV = function($row) {
-      return '"' . str_replace('"', '\"', $row) . '"';
+      return '"' . str_replace('"', '\"', (string)$row) . '"';
     };
     // add UTF-8 BOM (3 bytes, hex EF BB BF) at the start of the file for
     // Excel to automatically recognize the encoding