Some tests broke over the default None value.

mongoengine/fields.py

@@ -1,4 +1,5 @@
 from base import BaseField, ObjectIdField, ValidationError, get_document
+from queryset import DO_NOTHING
 from document import Document, EmbeddedDocument
 from connection import _get_db
 from operator import itemgetter
@@ -417,7 +418,7 @@ class ReferenceField(BaseField):
     access (lazily).
     """
 
-    def __init__(self, document_type, delete_rule=None, **kwargs):
+    def __init__(self, document_type, delete_rule=DO_NOTHING, **kwargs):
         if not isinstance(document_type, basestring):
             if not issubclass(document_type, (Document, basestring)):
                 raise ValidationError('Argument to ReferenceField constructor '