diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index c54d97a0b..23bee1401 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1398,12 +1398,13 @@ class UnitOfWork implements PropertyChangedListener
     public function addToIdentityMap($entity)
     {
         $classMetadata = $this->em->getClassMetadata(get_class($entity));
-        $idHash        = implode(' ', $this->entityIdentifiers[spl_object_hash($entity)]);
+        $identifier    = $this->entityIdentifiers[spl_object_hash($entity)];
 
-        if ($idHash === '') {
+        if (in_array(null, $identifier, true)) {
             throw ORMInvalidArgumentException::entityWithoutIdentity($classMetadata->name, $entity);
         }
 
+        $idHash    = implode(' ', $identifier);
         $className = $classMetadata->rootEntityName;
 
         if (isset($this->identityMap[$className][$idHash])) {