c# - JSON序列化错误

Question

我目前在调用我的 webapi 时遇到以下错误消息。

{"Message":"发生错误。","ExceptionMessage":"'ObjectContent`1' 类型未能序列化内容类型 'application/json; charset=utf-8' 的响应正文

下面是我的代码

public User getUser(string userName)
    {
        UserDAO userDao = db.GetDAO<UserDAO>();

        var user = userDao.GetWhere(x => x.Username == userName).ToList();
        if (user.Count < 1)
        {
            //the user is not yet in the database. 
            return new User { UserId = -1 };
        }

        return user.FirstOrDefault();
    }