diff --git a/src/Repository.php b/src/Repository.php
index 37e2c0c..2d28929 100644
--- a/src/Repository.php
+++ b/src/Repository.php
@@ -319,6 +319,10 @@ class Repository implements RepositoryInterface
                 }
             }
 
+            if (is_null($e1) || is_null($e2)) {
+                return $e1 !== $e2;
+            }
+
             return (string) $e1 !== (string) $e2;
         });
     }