SSRS Security extension - how to redirect login from inside GetUserInfo API

[TestO2015]

I'm writing an implementation of the SSRS Custom Security Extension and need to redirect requests to a login page from inside the IAuthenticationExtension2.GetUserInfo(...) API.

I've tried calling HttpContext.Current.Response.Redirect() however I receive an error from the Report Server:

The Authentication Extension threw an unexpected exception or returned a value that is not valid: identity==null. (rsAuthorizationTokenInvalidOrExpired)

Any ideas? Thanks.