If FormsAuthentication Ticket is set, why doesn't User.IsInRole(“Admin”) work?

In the debugger, if I dig into the User object, I can see the current member's UserData property, ((System.Web.Security.FormsIdentity(User.Identity)).Ticket.UserData, has "admin" in it.

User.Identity.IsAuthenticated works but User.IsInRole("admin") returns false.

If "admin" is in the UserData property, then why doesn't User.IsInRole("admin") return true?

In my login method I have the authentication ticket set up as follows:

 FormsAuthenticationTicket _ticket = new FormsAuthenticationTicket(1, lUserName.Text, DateTime.Now, DateTime.Now.AddMonths(1), chk_remember.Checked, Role, FormsAuthentication.FormsCookiePath);
        string encTicket = FormsAuthentication.Encrypt(_ticket);
        HttpCookie _cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
        if (chk_remember.Checked)
            _cookie.Expires = DateTime.Now.AddMonths(1);
        Response.Cookies.Add(_cookie);

you need to put this code in your Global.asax

protected void Application_AuthenticateRequest(Object sender,
EventArgs e)
{
  if (HttpContext.Current.User != null)
  {
    if (HttpContext.Current.User.Identity.IsAuthenticated)
    {
     if (HttpContext.Current.User.Identity is FormsIdentity)
     {
        FormsIdentity id =
            (FormsIdentity)HttpContext.Current.User.Identity;
        FormsAuthenticationTicket ticket = id.Ticket;

        // Get the stored user-data, in this case, our roles
        string userData = ticket.UserData;
        string[] roles = userData.Split(',');
        HttpContext.Current.User = new GenericPrincipal(id, roles);
     }
    }
  }
}

for more information you can see this link form authentication