Authentication returns null

If there is any stored value in ss-pid and session is timed out [Authenticate] attribute treats user as unauthenticated (expected).
When we try to authenticate the user again with value in ss-pid (cookie is populated when user clicks on remember me) through AuthenticateService,
response is always null. However if we clear the value of ss-pid, it’s working fine.

using (var authService = ResolveService<AuthenticateService>())
                            var response = authService.Authenticate(new Authenticate
                                provider = CredentialsAuthProvider.Name,
                                UserName = viewModel.UserName,
                                Password = viewModel.Password,
                                RememberMe = false

                            // add ASP.NET auth cookie
                            FormsAuthentication.SetAuthCookie(viewModel.UserName, false);

Any guidance is appreciated - Thanks!

If you’re already authenticated authService.Authenticate() will return null to indicate authentication was skipped as no authentication was required.