I am using ELMAH to log unhandled exceptions in an ASP.NET Webforms application. Logging is working fine.

I want to pass the ELMAH error log id to a custom error page that will give the user the ability to email an administrator about the error. I have followed the advice from this answer. Here is my global.asax code:

void ErrorLog_Logged(object sender, ErrorLoggedEventArgs args)
{        
    Session[StateKeys.ElmahLogId] = args.Entry.Id;

    // this doesn't work either:
    // HttpContext.Current.Items[StateKeys.ElmahLogId] = args.Entry.Id;
}

But, on the Custom error page, the session variable reference and HttpContext.Current.Items are giving me a NullReference exception. How can I pass the ID to my custom error page?