How can I fetch a list of objects through Restplus?

I try to use RESTplus to fetch objects. My implementation follows the Restplus tutorial. I managed to fetch a single object. However, I can not fetch a list with all objects.

This is a minimal implementation to reproduce the shortcoming:

from flask import Flask
from flask_restplus import Api, Resource, fields
from mongoengine import Document, connect
from mongoengine.fields import *

app = Flask(__name__)

connect()

api = Api(app)

user_model = api.model('user', {'name': fields.String})
user_model_list = api.model('User List', {'users':fields.List(fields.Nested(user_model))})


class User(Document):
    name = StringField()

@api.route('/Users')
class UserApi(Resource):
    @api.marshal_with(user_model_list)
    def get(self):
        return User.objects().all()

@api.route('/User')
class UserApi(Resource):
    @api.marshal_with(user_model)
    def get(self):
        return User.objects().first()

# Insert data into database
@app.before_first_request
def init():
    User(name='John').save()
    User(name='Sue').save()
    User(name='Li').save()


if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000

This is the output of GET http://127.0.0.1:5000/Users

{
    "users": null
}

The problem in my approach was that the MongoEngine returns a mongoengine.queryset.queryset.QuerySet ant not a list. After converting the QuerySet to a List, it worked even without as_list=True or marshal_list_with().

@api.route('/User')
class UserApi(Resource):
    @api.marshal_with(user_model)
    def get(self):
        return list(User.objects().first())