在我的登录方法中，对于行Startup.OAuthBearerOptions.AccessTokenFormat.Protect(ticket); AccessTokenFormat为null。任何想法为什么？

我的帐户控制器：

[RoutePrefix("api")]
public class AccountController : ApiController
{        
    public AccountController() {}

    // POST api/login
    [HttpPost]
    [Route("login")]
    public HttpResponseMessage Login(int id,string pwd)
    {
        if (id > 0) // testing - not authenticating right now
        {
            var identity = new ClaimsIdentity(Startup.OAuthBearerOptions.AuthenticationType);
            identity.AddClaim(new Claim(ClaimTypes.Name,id.ToString()));
            AuthenticationTicket ticket = new AuthenticationTicket(identity,new AuthenticationProperties());
            var currentUtc = new SystemClock().UtcNow;
            ticket.Properties.IssuedUtc = currentUtc;
            ticket.Properties.ExpiresUtc = currentUtc.Add(TimeSpan.FromMinutes(30));
            var token = Startup.OAuthBearerOptions.AccessTokenFormat.Protect(ticket);
            return new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new ObjectContent<object>(new
                {
                    UserName = id.ToString(),AccessToken = token
                },Configuration.Formatters.JsonFormatter)
            };
        }

        return new HttpResponseMessage(HttpStatusCode.BadRequest);
    }

    // POST api/token
    [Route("token")]
    [HttpPost]
    public HttpResponseMessage Token(int id,string pwd)
    {
        // Never reaches here. Do I need this method?
        return new HttpResponseMessage(HttpStatusCode.OK);
    }
}

启动类：

public class Startup
{
    private static readonly ILog _log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
    public static OAuthBearerAuthenticationOptions OAuthBearerOptions { get; private set; }
    public static OAuthAuthorizationServerOptions OAuthOptions { get; private set; }
    public static Func<MyUserManager> UserManagerFactory { get; set; }
    public static string PublicClientId { get; private set; }

    static Startup()
    {
        PublicClientId = "MyWeb";

        UserManagerFactory = () => new MyUserManager(new UserStore<MyIdentityUser>());

        OAuthBearerOptions = new OAuthBearerAuthenticationOptions();

        OAuthOptions = new OAuthAuthorizationServerOptions
        {
            TokenEndpointPath = new PathString("/api/token"),Provider = new MyWebOAuthProvider(PublicClientId,UserManagerFactory),AuthorizeEndpointPath = new PathString("/api/login"),AccessTokenExpireTimeSpan = TimeSpan.FromDays(14),AllowInsecureHttp = true
        };
    }

    public void Configuration(IAppBuilder app)
    {         
        // Enable the application to use bearer tokens to authenticate users
        app.USEOAuthBearerTokens(OAuthOptions);
        app.UseCookieAuthentication(new CookieAuthenticationOptions
        {
            AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,LoginPath = new PathString("/api/login")
        });

        // Configure Web API to use only bearer token authentication.
        var config = GlobalConfiguration.Configuration;            
        config.SuppressDefaultHostAuthentication();
        config.Filters.Add(new HostAuthenticationFilter(OAuthBearerOptions.AuthenticationType));

        app.UseWebApi(config);                          
    }
}

MyIdentityUser只添加一个额外的属性：

public class MyIdentityUser : IdentityUser
{
    public int SecurityLevel { get; set; }
}

MyUserManager将自定义用户身份验证方法调用到内部服务器：

public class MyUserManager : UserManager<MyIdentityUser>
{
    public MyUserManager(IUserStore<MyIdentityUser> store) : base(store) { }

    public MyIdentityUser ValidateUser(int id,string pwd)
    {
        LoginIdentityUser user = null;

        if (MyApplication.ValidateUser(id,pwd))
        {
            // user = ??? - not yet implemented
        }

        return user;
    }
}

MyWebOAuthProvider(我从SPA模板中获取，只有GrantResourceOwnerCredentials已经被更改)：

public class MyWebOAuthProvider : OAuthAuthorizationServerProvider
{
    private readonly string _publicClientId;
    private readonly Func<MyUserManager> _userManagerFactory;

    public MyWebOAuthProvider(string publicClientId,Func<MyUserManager> userManagerFactory)
    {
        if (publicClientId == null)
        {
            throw new ArgumentNullException("publicClientId");
        }

        if (userManagerFactory == null)
        {
            throw new ArgumentNullException("userManagerFactory");
        }

        _publicClientId = publicClientId;
        _userManagerFactory = userManagerFactory;
    }

    public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {
        using (MyUserManager userManager = _userManagerFactory())
        {
            MyIdentityUser user = null;
            var ctx = context as MyWebOAuthGrantResourceOwnerCredentialsContext;

            if (ctx != null)
            {
                user = userManager.ValidateUser(ctx.Id,ctx.Pwd);
            }                

            if (user == null)
            {
                context.SetError("invalid_grant","The user name or password is incorrect.");
                return;
            }

            ClaimsIdentity oAuthIdentity = await userManager.CreateIdentityAsync(user,context.Options.AuthenticationType);
            ClaimsIdentity cookiesIdentity = await userManager.CreateIdentityAsync(user,CookieAuthenticationDefaults.AuthenticationType);
            AuthenticationProperties properties = CreateProperties(user.UserName);
            AuthenticationTicket ticket = new AuthenticationTicket(oAuthIdentity,properties);
            context.Validated(ticket);
            context.Request.Context.Authentication.SignIn(cookiesIdentity);
        }
    }

    public override Task TokenEndpoint(OAuthTokenEndpointContext context)
    {
        ...  // unchanged from SPA template
    }

    public override Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
    {
        ...  // unchanged from SPA template
    }

    public override Task ValidateClientRedirectUri(OAuthValidateClientRedirectUriContext context)
    {
        ...  // unchanged from SPA template
    }

    public static AuthenticationProperties CreateProperties(string userName)
    {
        ...  // unchanged from SPA template
    }
}

MyWebOAuthGrantResourceOwnerCredientialsContext：

public class MyWebOAuthGrantResourceOwnerCredentialsContext : OAuthGrantResourceOwnerCredentialsContext
{
    public MyWebOAuthGrantResourceOwnerCredentialsContext (IOwinContext context,OAuthAuthorizationServerOptions options,string clientId,string userName,string password,IList<string> scope)
        : base(context,options,clientId,userName,password,scope)
    { }

    public int Id { get; set; }        
    public string Pwd { get; set; }
}

如何设置AccessTokenFormat？是我设定的正确吗？我不会对任何外部服务进行身份验证，只是传统的内部服务器。
谢谢。