FormsAuthentication.SignOut throwing NullReferenceException

This problem seems related to this post, but I was not able to infer a solution from the thread.

I noticed this code in an application I inherited (after noting in a log file that an exception was being eaten):

    protected void Session_End(object sender, EventArgs e)
    {
        try
        {
            FormsAuthentication.SignOut();
            FormsAuthentication.RedirectToLoginPage();
            //if (this.Context.Handler is IRequiresSessionState || this.Context.Handler is IReadOnlySessionState)
            //{
            //    FormsAuthentication.SignOut();
            //    FormsAuthentication.RedirectToLoginPage();
            //}
        }
        catch (Exception ex)
        {
            this.GetType().GetLogger().Error(ex);
        }
    }

I am wondering a few things. First, how is SignOut throwing a null reference exception? Is it an exceptional case, or am I doing something inherently wrong in my program? Next, what should I be testing against to head-off this exception before it is thrown?

15:51:57,288 [13] ERROR ASP.global_asax - System.NullReferenceException: Object reference not set to an instance of an object. at System.Web.Security.FormsAuthentication.SignOut() at MvcApplication.Session_End

Thanks