diff --git a/src/Plugin/inmail/Deliverer/FetcherBase.php b/src/Plugin/inmail/Deliverer/FetcherBase.php
index a0e5ebf..0f0c56e 100644
--- a/src/Plugin/inmail/Deliverer/FetcherBase.php
+++ b/src/Plugin/inmail/Deliverer/FetcherBase.php
@@ -115,7 +115,7 @@ abstract class FetcherBase extends DelivererBase implements FetcherInterface {
    * Handles submit call of "Test connection" button.
    */
   public function submitTestConnection(array $form, FormStateInterface $form_state) {
-    // Implement it in the subclass.
+    throw new \Exception('Implement submitTestConnection() method in a subclass.');
   }
 
   /**