diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index 7044ce57e..b2c1c6f4b 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1796,7 +1796,7 @@ class UnitOfWork implements PropertyChangedListener
      * @throws ORMInvalidArgumentException If the entity instance is NEW.
      * @throws EntityNotFoundException
      */
-    private function doMerge($entity, array &$visited, $prevManagedCopy = null, $assoc = null)
+    private function doMerge($entity, array &$visited, $prevManagedCopy = null, array $assoc = [])
     {
         $oid = spl_object_hash($entity);