diff --git a/mongoengine/dereference.py b/mongoengine/dereference.py
index 997b7858..e1b0a038 100644
--- a/mongoengine/dereference.py
+++ b/mongoengine/dereference.py
@@ -115,7 +115,7 @@ class DeReference(object):
         object_map = {}
         for col, dbrefs in self.reference_map.iteritems():
             keys = object_map.keys()
-            refs = list(set([dbref for dbref in dbrefs if str(dbref) not in keys]))
+            refs = list(set([dbref for dbref in dbrefs if dbref.encode('utf-8') not in keys]))
             if hasattr(col, 'objects'):  # We have a document class for the refs
                 references = col.objects.in_bulk(refs)
                 for key, doc in references.iteritems():