注册外部登录时CreateUserAsync失败

我正在使用asp.net mvc 5模板处理单页应用程序并注册外部登录(在此实例中为google)失败并出现验证异常 - "UserId字段是必需的".有问题的代码:

// POST api/Account/RegisterExternal [OverrideAuthentication]
[HostAuthentication(DefaultAuthenticationTypes.ExternalBearer)]
[Route("RegisterExternal")]
public async Task RegisterExternal(RegisterExternalBindingModel model)
{
    try
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        ExternalLoginData externalLogin = ExternalLoginData.FromIdentity(User.Identity as ClaimsIdentity);

        if (externalLogin == null)
        {
            return InternalServerError();
        }

        IdentityUser user = new IdentityUser//(model.UserName);
        {
             UserName = model.UserName
        };
        user.Logins.Add(new IdentityUserLogin
        {
             LoginProvider = externalLogin.LoginProvider,
             ProviderKey = externalLogin.ProviderKey
        });
        IdentityResult result = await UserManager.CreateAsync(user);
        IHttpActionResult errorResult = GetErrorResult(result);

        if (errorResult != null)
        {
           return errorResult;
        }

        return Ok();
    }
    catch (Exception exception)
    {
        throw;
    }
}

我很感激帮助