1. A method for performing authentication in a communication system comprising an authentication server (AS), and a user profile store storing user profiles for users of the communication system, the method comprising: transmitting from the authentication server to the user profile store a request for the user profile of a user; receiving at the AS a response to the request; determining whether the response is indicative of an error; and if the response is indicative of an error, transmitting from the authentication server to the user profile store a message of a type such as to trigger the user profile store to perform a location update procedure in respect of the user.