User.Identity.GetUserId() always returns null in controller action

I needed to use current logged in user id in controller. It always returns null. I am already logged in. Here's my code.

if (string.IsNullOrEmpty(userId)) userId = User.Identity.GetUserId();

I tried adding the reference

 using Microsoft.AspNet.Identity;

I also tried adding a claim

public const string UserId = "http://schemas.xmlsoap.org/ws/2014/03/mystuff/claims/UserId";
userIdentity.AddClaim(new Claim(CustomClaimTypes.UserId, Id));

When I try to retrieve the claim, it gives null as well.

(identity as ClaimsIdentity).FirstOrNull(CustomClaimTypes.UserId);

Any help would be appreciated.

Additional Info requested in comments:

It's the normal login provided by aspnetuser.

        var result = await SignInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, shouldLockout: false);
        switch (result)
        {
            case SignInStatus.Success:
                return RedirectToLocal(returnUrl);
            case SignInStatus.LockedOut:
                return View("Lockout");
            case SignInStatus.RequiresVerification:
                return RedirectToAction("SendCode", new { ReturnUrl = returnUrl, RememberMe = model.RememberMe });
            case SignInStatus.Failure:
            default:
                ModelState.AddModelError("", "Invalid login attempt.");
                return View(model);
        }

Request.IsAuthenticated returns true.

Claims added here.

    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
    {
        // Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        // Add custom user claims here
        userIdentity.AddClaim(new Claim(CustomClaimTypes.UserId, Id));

        return userIdentity;
    }

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