Catching errors in Global.asax

Question

I have the following in my Global.aspx which is meant for handling errors:

void Application_Error(object sender, EventArgs e)
{
    Exception exception = Server.GetLastError();
    if (exception != null)
    {
        //Log
        if (HttpContext.Current.Server != null)
        {
           HttpContext.Current.Server.Transfer("/siteerror.aspx");
        }
    }
}

This works for the most part, but sometimes does not get into Server.Transfer. For some reason HttpContext.Current.Server is null. I figured out where this happens: when errors occur in a user control and in my business logic classes. Am I missing something here?

Thanks