diff --git a/mongoengine/queryset.py b/mongoengine/queryset.py
index 9f24dea5..e2273620 100644
--- a/mongoengine/queryset.py
+++ b/mongoengine/queryset.py
@@ -522,6 +522,7 @@ class QuerySet(object):
                     raise InvalidQueryError('Cannot resolve field "%s"'
                                             % field_name)
             fields.append(field)
+            field = None
         return fields
 
     @classmethod