diff --git a/PHPUnit/Framework/Comparator/MockObject.php b/PHPUnit/Framework/Comparator/MockObject.php
index a1194c709e2..5c6478c4389 100644
--- a/PHPUnit/Framework/Comparator/MockObject.php
+++ b/PHPUnit/Framework/Comparator/MockObject.php
@@ -81,7 +81,6 @@ protected function toArray($object)
         $array = parent::toArray($object);
 
         unset($array['__phpunit_invocationMocker']);
-        unset($array['__phpunit_id']);
 
         return $array;
     }