diff --git a/Handler/AbstractExceptionHandler.php b/Handler/AbstractExceptionHandler.php
index 4dd594e..15cb0fb 100644
--- a/Handler/AbstractExceptionHandler.php
+++ b/Handler/AbstractExceptionHandler.php
@@ -311,7 +311,7 @@ public function onKernelResponse(ResponseEvent $responseObject): void
     }
   }
 
-  protected function generateHash(string $url, string $exceptionMessage): string
+  protected function generateHash(string $url, ?string $exceptionMessage): string
   {
     return md5($url . $exceptionMessage);
   }