使用AFNetworking读取JSON Web API 2 InvalidModelStateResult结果

我正在尝试处理正在构建的iOS应用程序中的ModelState问题。当我想注册用户时，我会将他们的详细信息发送到以下方法：

    // POST api/Account/Register
    [AllowAnonymous]
    [Route("Register")]
    public async Task<IHttpActionResult> Register(RegisterBindingModel model)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        var user = new ApplicationUser() { UserName = model.Email, Email = model.Email };

        IdentityResult result = await UserManager.CreateAsync(user, model.Password);

        if (!result.Succeeded)
        {
            return GetErrorResult(result);
        }

        return Ok();
    }

我在iOS应用程序（Swift）中使用以下方法读取了JSON响应：

func register(user:RegisterModel, successClosure: (() -> ())?, failureClosure: (() -> ())?) {
    let manager  = AFHTTPRequestOperationManager()
    let registerUrl = XXXX

    manager.POST( registerUrl,
        parameters: ["email": user.email, "password": user.password, "confirmpassword": user.passwordConfirmation],
        success: { (operation: AFHTTPRequestOperation!,responseObject: AnyObject!) in
            println("register success")
            if let succ = successClosure {
                succ()
            }
        },
        failure: { (operation: AFHTTPRequestOperation!,error: NSError!) in
            println("register fail")
            println(error)
            if let fail = failureClosure {
                fail()
            }
    })
}

收到错误（BadRequest(ModelState)）时，输出如下：

Error Domain=com.alamofire.error.serialization.response Code=-1011 "Request failed: bad request (400)" UserInfo=0x7f8663e3e890 {com.alamofire.serialization.response.error.response=<NSHTTPURLResponse: 0x7f8663e3cba0> { URL: http://192.168.0.71:8888/api/Account/Register } { status code: 400, headers {
"Cache-Control" = "no-cache";
"Content-Length" = 227;
"Content-Type" = "application/json; charset=utf-8";
Date = "Sun, 17 Aug 2014 00:31:57 GMT";
Expires = "-1";
Pragma = "no-cache";
Server = "Microsoft-IIS/8.0";
"X-AspNet-Version" = "4.0.30319";
"X-Powered-By" = "ASP.NET";
"X-SourceFiles" = "=?UTF-8?B?WjpcUHJvamVjdHNcTGFybnNmaXRcTGFybnNmaXRcYXBpXEFjY291bnRcUmVnaXN0ZXI=?=";
} }, NSLocalizedDescription=Request failed: bad request (400), NSErrorFailingURLKey=http://192.168.0.71:8888/api/Account/Register}

Web API 2是否没有向我发送ModelState无效的原因？还是我错误地访问了错误响应？最终，我想知道ModelState无效的原因是什么。