This method is used to Redirects the browser to the login URL. But sometime this function not works as expected and user is not sent to login page. As defined in MSDN documentation “Unlike the HttpResponse.Redirect method, this method does not end the request by calling HttpResponse.End. This means that code that follows the RedirectToLoginPage method call will run.” If FormsAuthentication .RedirectToLoginPage() method not working as expected that means code in the Page life cycle after your call to RedirectToLoginPage is running, and throwing a NullReferenceException. Solution: Write Response.End() after FormsAuthentication .RedirectToLoginPage(), and it will work fine then. Note: If you want to know details about what the actual response contains when the method is called and why there is Null reference exception, you can use a trace tool such as Fiddler to see.