diff --git a/lib/API/JSON/API.php b/lib/API/JSON/API.php
index 081818c27c..22edf3446e 100644
--- a/lib/API/JSON/API.php
+++ b/lib/API/JSON/API.php
@@ -58,13 +58,13 @@ class API {
     $this->setRequestData($_POST);
 
     $ignoreToken = (
-      Setting::getValue('re_captcha.enabled') && 
-      $this->_request_endpoint === 'subscribers' && 
+      Setting::getValue('re_captcha.enabled') &&
+      $this->_request_endpoint === 'subscribers' &&
       $this->_request_method === 'subscribe'
-    ); 
+    );
 
     if(!$ignoreToken && $this->checkToken() === false) {
-      $error_message = __('Sorry, but we couldn\'t connect to the MailPoet server. Please refresh the web page and try again.', 'mailpoet');
+      $error_message = __("Sorry, but we couldn't connect to the MailPoet server. Please refresh the web page and try again.", 'mailpoet');
       $error_response = $this->createErrorResponse(Error::UNAUTHORIZED, $error_message, Response::STATUS_UNAUTHORIZED);
       return $error_response->send();
     }
diff --git a/lib/API/JSON/v1/Subscribers.php b/lib/API/JSON/v1/Subscribers.php
index bf1850b72a..1a1e4cec91 100644
--- a/lib/API/JSON/v1/Subscribers.php
+++ b/lib/API/JSON/v1/Subscribers.php
@@ -14,6 +14,7 @@ use MailPoet\Models\Subscriber;
 use MailPoet\Newsletter\Scheduler\Scheduler;
 use MailPoet\Segments\BulkAction;
 use MailPoet\Segments\SubscribersListings;
+use MailPoet\Subscribers\RequiredCustomFieldValidator;
 use MailPoet\Subscribers\Source;
 use MailPoet\Subscription\Throttling as SubscriptionThrottling;
 use MailPoet\WP\Hooks;
@@ -104,7 +105,7 @@ class Subscribers extends APIEndpoint {
         'body' => array(
           'secret' => $recaptcha['secret_token'],
           'response' => $res
-        ) 
+        )
       ));
       if(is_wp_error($res)) {
         return $this->badRequest(array(
@@ -121,6 +122,13 @@ class Subscribers extends APIEndpoint {
 
     $data = $this->deobfuscateFormPayload($data);
 
+    try {
+      $validator = new RequiredCustomFieldValidator();
+      $validator->validate($data);
+    } catch (\Exception $e) {
+      return $this->badRequest([APIError::BAD_REQUEST => $e->getMessage()]);
+    }
+
     $segment_ids = (!empty($data['segments'])
       ? (array)$data['segments']
       : array()
diff --git a/lib/API/MP/v1/API.php b/lib/API/MP/v1/API.php
index 8c7d5a512c..31f800c230 100644
--- a/lib/API/MP/v1/API.php
+++ b/lib/API/MP/v1/API.php
@@ -6,6 +6,7 @@ use MailPoet\Models\Segment;
 use MailPoet\Models\Subscriber;
 use MailPoet\Models\SubscriberSegment;
 use MailPoet\Newsletter\Scheduler\Scheduler;
+use MailPoet\Subscribers\RequiredCustomFieldValidator;
 use MailPoet\Subscribers\Source;
 use MailPoet\Tasks\Sending;
 
@@ -164,6 +165,9 @@ class API {
     // if some required default fields are missing, set their values
     $default_fields = Subscriber::setRequiredFieldsDefaultValues($default_fields);
 
+    $validator = new RequiredCustomFieldValidator();
+    $validator->validate($custom_fields);
+
     // add subscriber
     $new_subscriber = Subscriber::create();
     $new_subscriber->hydrate($default_fields);
diff --git a/lib/Subscribers/RequiredCustomFieldValidator.php b/lib/Subscribers/RequiredCustomFieldValidator.php
new file mode 100644
index 0000000000..29283c1e0e
--- /dev/null
+++ b/lib/Subscribers/RequiredCustomFieldValidator.php
@@ -0,0 +1,55 @@
+<?php
+
+namespace MailPoet\Subscribers;
+
+use MailPoet\Models\CustomField;
+
+class RequiredCustomFieldValidator {
+
+  /**
+   * @param array $data
+   *
+   * @throws \Exception
+   */
+  public function validate(array $data) {
+    $all_custom_fields = $this->getCustomFields();
+    foreach($all_custom_fields as $custom_field_id => $custom_field_name) {
+      if($this->isCustomFieldMissing($custom_field_id, $data)) {
+        throw new \Exception(
+          __(sprintf('Missing value for custom field "%s"', $custom_field_name), 'mailpoet')
+        );
+      }
+    }
+  }
+
+  private function isCustomFieldMissing($custom_field_id, $data) {
+    if(!array_key_exists($custom_field_id, $data) && !array_key_exists('cf_' . $custom_field_id, $data)) {
+      return true;
+    }
+    if(isset($data[$custom_field_id]) && !$data[$custom_field_id]) {
+      return true;
+    }
+    if(isset($data['cf_' . $custom_field_id]) && !$data['cf_' . $custom_field_id]) {
+      return true;
+    }
+    return false;
+  }
+
+  private function getCustomFields() {
+    $result = [];
+
+    $required_custom_fields = CustomField::findMany();
+
+    foreach($required_custom_fields as $custom_field) {
+      if(is_serialized($custom_field->params)) {
+        $params = unserialize($custom_field->params);
+        if(is_array($params) && isset($params['required']) && $params['required']) {
+          $result[$custom_field->id] = $custom_field->name;
+        }
+      }
+    }
+
+    return $result;
+  }
+
+}
diff --git a/tests/unit/API/JSON/v1/SubscribersTest.php b/tests/unit/API/JSON/v1/SubscribersTest.php
index 827ddebdac..9ae432479f 100644
--- a/tests/unit/API/JSON/v1/SubscribersTest.php
+++ b/tests/unit/API/JSON/v1/SubscribersTest.php
@@ -6,6 +6,7 @@ use Codeception\Util\Fixtures;
 use MailPoet\API\JSON\v1\Subscribers;
 use MailPoet\API\JSON\Response as APIResponse;
 use MailPoet\Form\Util\FieldNameObfuscator;
+use MailPoet\Models\CustomField;
 use MailPoet\Models\Form;
 use MailPoet\Models\Newsletter;
 use MailPoet\Models\NewsletterOption;
@@ -478,6 +479,21 @@ class SubscribersTest extends \MailPoetTest {
     Setting::setValue('re_captcha', array());
   }
 
+  function testItCannotSubscribeWithoutMandatoryCustomField() {
+    CustomField::createOrUpdate([
+      'name' => 'custom field',
+      'type' => 'text',
+      'params' => ['required' => '1']
+    ]);
+    $router = new Subscribers();
+    $response = $router->subscribe(array(
+      $this->obfuscatedEmail => 'toto@mailpoet.com',
+      'form_id' => $this->form->id,
+      $this->obfuscatedSegments => array($this->segment_1->id, $this->segment_2->id)
+    ));
+    expect($response->status)->equals(APIResponse::STATUS_BAD_REQUEST);
+  }
+
   function testItCanSubscribeWithoutSegmentsIfTheyAreSelectedByAdmin() {
     $form = $this->form->asArray();
     $form['settings']['segments_selected_by'] = 'admin';
@@ -642,5 +658,6 @@ class SubscribersTest extends \MailPoetTest {
     \ORM::raw_execute('TRUNCATE ' . Subscriber::$_table);
     \ORM::raw_execute('TRUNCATE ' . SubscriberSegment::$_table);
     \ORM::raw_execute('TRUNCATE ' . SubscriberIP::$_table);
+    \ORM::raw_execute('TRUNCATE ' . CustomField::$_table);
   }
 }
diff --git a/tests/unit/API/MP/APITest.php b/tests/unit/API/MP/APITest.php
index 71c6cc9ea6..919b6663c7 100644
--- a/tests/unit/API/MP/APITest.php
+++ b/tests/unit/API/MP/APITest.php
@@ -301,6 +301,21 @@ class APITest extends \MailPoetTest {
     expect($result['source'])->equals('api');
   }
 
+  function testItChecksForMandatoryCustomFields() {
+    CustomField::createOrUpdate([
+      'name' => 'custom field',
+      'type' => 'text',
+      'params' => ['required' => '1']
+    ]);
+
+    $subscriber = array(
+      'email' => 'test@example.com',
+    );
+
+    $this->setExpectedException('Exception');
+    API::MP(self::VERSION)->addSubscriber($subscriber);
+  }
+
   function testItSubscribesToSegmentsWhenAddingSubscriber() {
     $API = Stub::makeEmptyExcept(
       new \MailPoet\API\MP\v1\API(),
diff --git a/tests/unit/Subscribers/RequiredCustomFieldValidatorTest.php b/tests/unit/Subscribers/RequiredCustomFieldValidatorTest.php
new file mode 100644
index 0000000000..ad3ca79373
--- /dev/null
+++ b/tests/unit/Subscribers/RequiredCustomFieldValidatorTest.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace MailPoet\Subscribers;
+
+use MailPoet\Models\CustomField;
+
+class RequiredCustomFieldValidatorTest extends \MailPoetTest {
+
+  private $custom_field;
+
+  function _before() {
+    \ORM::raw_execute('TRUNCATE ' . CustomField::$_table);
+    $this->custom_field = CustomField::createOrUpdate([
+      'name' => 'custom field',
+      'type' => 'text',
+      'params' => ['required' => '1']
+    ]);
+  }
+
+  function testItValidatesDataWithoutCustomField() {
+    $validator = new RequiredCustomFieldValidator();
+    $this->setExpectedException('Exception');
+    $validator->validate([]);
+  }
+
+  function testItValidatesDataWithCustomFieldPassedAsId() {
+    $validator = new RequiredCustomFieldValidator();
+    $validator->validate([$this->custom_field->id() => 'value']);
+  }
+
+  function testItValidatesDataWithCustomFieldPassedAsCFId() {
+    $validator = new RequiredCustomFieldValidator();
+    $validator->validate(['cf_' . $this->custom_field->id() => 'custom field']);
+  }
+
+  function testItValidatesDataWithEmptyCustomField() {
+    $validator = new RequiredCustomFieldValidator();
+    $this->setExpectedException('Exception');
+    $validator->validate([$this->custom_field->id() => '']);
+  }
+
+  function testItValidatesDataWithEmptyCustomFieldAsCFId() {
+    $validator = new RequiredCustomFieldValidator();
+    $this->setExpectedException('Exception');
+    $validator->validate(['cf_' . $this->custom_field->id() => '']);
+  }
+
+}
\ No newline at end of file