Unable to cast custom Identity when using Application_AuthenticateRequest

When I add
 if (Context.User != null)
            {
                if (Context.User.Identity.IsAuthenticated)
                {
                    if (!(Context.User is AdministratorPrincipal))
                    {
                       Admin.Web.Principal.AdministratorPrincipal newUser = new Admin.Web.Principal.AdministratorPrincipal(Context.User.Identity.Name);
           ...