diff --git a/src/DataDog/AuditBundle/EventSubscriber/AuditSubscriber.php b/src/DataDog/AuditBundle/EventSubscriber/AuditSubscriber.php
index 45ed1ec..47d3254 100644
--- a/src/DataDog/AuditBundle/EventSubscriber/AuditSubscriber.php
+++ b/src/DataDog/AuditBundle/EventSubscriber/AuditSubscriber.php
@@ -381,7 +381,7 @@ protected function diff(EntityManager $em, $entity, array $ch)
                 ];
             }
         }
-        return $diff;
+        return json_encode($diff);
     }
 
     protected function assoc(EntityManager $em, $association = null)