i keep on getting the message couldn't create a valid password

from flask import Blueprint

from flask.ext.restful import Resource, Api, reqparse, marshal_with, fields

import models

user_fields = {
    'username': fields.String
}


class UserList(Resource):
    def __init__(self):
        self.reqparse = reqparse.RequestParser()
        self.reqparse.add_argument(
            'username',
            required=True,
            location=['form', 'json']
        )
        self.reqparse.add_argument(
            'password',
            required=True,
            location=['form', 'json']
        )
        self.reqparse.add_argument(
            'confirm_password',
            required=True,
            location=['form', 'json']
        )

        super().__init__()

@marshal_with(user_fields)
def post(self):
    args = self.reqparse.parse_args()
    if args.get('password') == args.get('confirm_password'):
        user = models.User.create(**args)
        return marshal(user, user_fields), 201
    return make_response(json.dumps({'error': 'Password and confirm_password do not match'}), 400)


users_api = Blueprint('resources.users', __name__)
api = Api(users_api)
api.add_resource(UserList, '/api/v1/users')