SSO logoff not clearing the session

I am implementing SSO with using CAS.  We need to have a logoff and my logoff is not clearing the session.
ASP.Net 4.0
Here is the code for  logoff page:
                    // On PAge Load Explicitly clear the session vars:            Session["MYVAR1"] = null;            Session["MYVAR2"] = null;            .            .                        FormsAuthentication.SignOut();             Session.Clear();         ...