got "Internal server Error" when trying to query for _User 

We've set up the parse-server with Heroku & MongoLab and migrate the user successfully. But when we try to query for User, we got the "Internal server error." message. 

```
    var User = Parse.Object.extend("_User");
    var query = new Parse.Query(User);
    query.find( ... ) 
```

Anyone could give us a hint ? 
