How to specify that an argument is optional in flask-restful

I have code like this:

def delete(self, rid):
    parser = reqparse.RequestParser()
    parser.add_argument('rating', default=2, type=int, help='blablabla')
    args = parser.parse_args()

    rating = args['rating']
    ...
    return {'message': message}

This still asks me for the rating param, and throws 400 Bad Request if no exist.

Did I miss something?