diff --git a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
index 5be6c33..b31bdb9 100644
--- a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
@@ -72,7 +72,8 @@ public function on4xx(GetResponseForExceptionEvent $event) {
       $encoded_content = $this->serializer->serialize($exception, $format);
     }
     else {
-      $encoded_content = ['message' => $exception->getMessage()];
+      $content = ['message' => $exception->getMessage()];
+      $encoded_content = $this->serializer->serialize($content, $format);
     }
     $headers = $exception->getHeaders();