Change Primary Key for Users in ASP.NET Identity

To achieve int primary key I followed this article: https://www.asp.net/identity/overview/extensibility/change-primary-key-for-users-in-aspnet-identity

It compiles but on startup it gives me this error message:

An exception of type 'System.FormatException' occurred in mscorlib.dll but was not handled in user code
Additional information: Input string was not in a correct format.

Code:

    public void ConfigureAuth(IAppBuilder app)
    {
        // Configure the db context, user manager and signin manager to use a single instance per request
        app.CreatePerOwinContext(ApplicationDbContext.Create);
        app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
        app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);

        // Enable the application to use a cookie to store information for the signed in user
        // and to use a cookie to temporarily store information about a user logging in with a third party login provider
        // Configure the sign in cookie
        app.UseCookieAuthentication(new CookieAuthenticationOptions
        {
            AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
            LoginPath = new PathString("/Account/Login"),
            Provider = new CookieAuthenticationProvider
            {
                OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, ApplicationUser, int>(
                validateInterval: TimeSpan.FromMinutes(30),
                regenerateIdentityCallback: (manager, user) =>
                    user.GenerateUserIdentityAsync(manager),
================================================================================
                    getUserIdCallback: (id) => (id.GetUserId<int>()  <<===== THIS IS WHERE I GET THE ERROR MESSAGE))
================================================================================

            }
        });            
        app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);

        // Enables the application to temporarily store user information when they are verifying the second factor in the two-factor authentication process.
        app.UseTwoFactorSignInCookie(DefaultAuthenticationTypes.TwoFactorCookie, TimeSpan.FromMinutes(5));

        // Enables the application to remember the second login verification factor such as phone or email.
        // Once you check this option, your second step of verification during the login process will be remembered on the device where you logged in from.
        // This is similar to the RememberMe option when you log in.
        app.UseTwoFactorRememberBrowserCookie(DefaultAuthenticationTypes.TwoFactorRememberBrowserCookie);

        // Uncomment the following lines to enable logging in with third party login providers
        //app.UseMicrosoftAccountAuthentication(
        //    clientId: "",
        //    clientSecret: "");

        //app.UseTwitterAuthentication(
        //   consumerKey: "",
        //   consumerSecret: "");

        //app.UseFacebookAuthentication(
        //   appId: "",
        //   appSecret: "");

        //app.UseGoogleAuthentication(new GoogleOAuth2AuthenticationOptions()
        //{
        //    ClientId = "",
        //    ClientSecret = ""
        //});
    }

Does anyone know where is the problem?

Tetsuya Yamamoto is right. The problem was that I took a clean project, ran it, and registered as a user while the primary key was GUID. Then I performed all the changes needed in order for the primary key to become integer. Then, when I ran it the cookie was sent to the OnValidateIdentity and there it crashed since the primary key was GUID. I cleaned all cache and everything worked as expected.