Unable to cast object of type 'System.Web.Security.FormsIdentity' to type 'XXX'

I got following
        protected void Application_AuthenticateRequest(object sender, EventArgs e)
        {
            if (Request.IsAuthenticated)
            {

                var user = Membership.GetUser();

                var identity = new CustomIdentity(user.UserName, new Guid(user.ProviderUserKey.ToString()));

                var principal = new RolePrincipal(identity);
                System.Threading.Thread.CurrentPrincipal = principal;
            }

 &nbs ...