diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index 12cc7f2a..7729444e 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -770,7 +770,7 @@ class ReferenceField(BaseField):
     def to_mongo(self, document):
         if isinstance(document, DBRef):
             if not self.dbref:
-                return "%s" % DBRef.id
+                return DBRef.id
             return document
         elif not self.dbref and isinstance(document, basestring):
             return document