HttpContext.Current.User.Identity.Name is always string.Empty

Hi I use a custom MembershipProvider.

I want to know the current username during an application scenario, but when I try accessing HttpContext.Current.User.Identity.Name it always returns string.Empty.

if (Membership.ValidateUser(tbUsername.Text, tbPassword.Text))
{
    FormsAuthentication.SetAuthCookie(tbUsername.Text, true);
    bool x = User.Identity.IsAuthenticated; //true
    string y = User.Identity.Name; //""
    FormsAuthentication.RedirectFromLoginPage(tbUsername.Text, cbRememberMe.Checked);
}

Am I missing something?