diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index 73d9757de..f53ec67a7 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -2456,6 +2456,6 @@ class UnitOfWork implements PropertyChangedListener
         if ( ! is_object($object) ) {
             throw new InvalidArgumentException("Managed entity required");
         }
-        return $this->readOnlyObjects[spl_object_hash($object)];
+        return isset($this->readOnlyObjects[spl_object_hash($object)]);
     }
 }