diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index a2250f4c..f7344488 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -734,6 +734,9 @@ class GenericReferenceField(BaseField):
     def to_mongo(self, document):
         if document is None:
             return None
+        
+        if isinstance(document, (dict, SON)):
+            return document                         
 
         id_field_name = document.__class__._meta['id_field']
         id_field = document.__class__._fields[id_field_name]