diff --git a/flask_mongoengine/json.py b/flask_mongoengine/json.py
index 83637632..479e4e99 100644
--- a/flask_mongoengine/json.py
+++ b/flask_mongoengine/json.py
@@ -78,7 +78,7 @@ def default(obj):
                 (BaseDocument, QuerySet, CommandCursor, DBRef, ObjectId),
             ):
                 return _convert_mongo_objects(obj)
-            return super().default(obj)
+            return super(MongoEngineJSONProvider, MongoEngineJSONProvider).default(obj)
 
     return MongoEngineJSONProvider