在AspNetCore.Identity中的Google身份验证

开始使用AspNetCore。身份，但不能运行简单的例子，总是收到这样的异常:

处理请求时发生未处理的异常。InvalidOperationException:没有配置身份验证处理程序处理方案:google

Startup.cs

    public void ConfigureServices(IServiceCollection services)
    {
        // EF services
        services.AddEntityFramework()
            .AddEntityFrameworkSqlServer()
            .AddDbContext<MyContext>();
        // Identity services
        services.AddIdentity<IdentityUser, IdentityRole>()
            .AddEntityFrameworkStores<MyContext>()
            .AddDefaultTokenProviders();
        // MVC services
        services.AddMvc().AddJsonOptions(options => {
            options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
            options.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
            options.SerializerSettings.Converters = new JsonConverter[] { new StringEnumConverter(), new IsoDateTimeConverter() };
        });

Configure.cs

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        loggerFactory.AddConsole();
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        app.UseIdentity();
        app.UseCookieAuthentication();
        app.UseGoogleAuthentication(new GoogleOptions()
        {
            ClientId = "xxx",
            ClientSecret = "xxx",
            AutomaticChallenge = true,
            AutomaticAuthenticate = true
        });
        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller}/{action}/{id?}");
        });

AuthController.cs

    [HttpGet]
    [AllowAnonymous]
    [Route("ExternalLogin", Name = "ExternalLogin")]
    public IActionResult ExternalLogin(string provider, string returnUrl = null)
    {
        var redirectUrl = Url.Action("ExternalLoginCallback", "Auth", new { ReturnUrl = returnUrl });
        var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirectUrl);
        return new ChallengeResult(provider, properties);
    }

返回ChallengeResult后某处发生异常。我错过什么了吗?