diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index abea212c..a1638bf0 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -666,6 +666,9 @@ class ReferenceField(BaseField):
         return pymongo.dbref.DBRef(collection, id_)
 
     def prepare_query_value(self, op, value):
+        if value is None:
+            return None
+
         return self.to_mongo(value)
 
     def validate(self, value):
@@ -743,6 +746,9 @@ class GenericReferenceField(BaseField):
         return {'_cls': document._class_name, '_ref': ref}
 
     def prepare_query_value(self, op, value):
+        if value is None:
+            return None
+
         return self.to_mongo(value)
 
 
diff --git a/tests/fields.py b/tests/fields.py
index c95b544e..80a343e3 100644
--- a/tests/fields.py
+++ b/tests/fields.py
@@ -992,15 +992,29 @@ class FieldTest(unittest.TestCase):
         class Company(Document):
             name = StringField()
 
+        Product.drop_collection()
+        Company.drop_collection()
+
         ten_gen = Company(name='10gen')
         ten_gen.save()
         mongodb = Product(name='MongoDB', company=ten_gen)
         mongodb.save()
 
+        me = Product(name='MongoEngine')
+        me.save()
+
         obj = Product.objects(company=ten_gen).first()
         self.assertEqual(obj, mongodb)
         self.assertEqual(obj.company, ten_gen)
 
+        obj = Product.objects(company=None).first()
+        self.assertEqual(obj, me)
+
+        obj, created = Product.objects.get_or_create(company=None)
+        
+        self.assertEqual(created, False)
+        self.assertEqual(obj, me)
+
     def test_reference_query_conversion(self):
         """Ensure that ReferenceFields can be queried using objects and values
         of the type of the primary key of the referenced object.