document.referrer blank when using back button

Question

I am using document.referrer in javascript to check where the user has come from. At the moment I am looking into what happens if the user is on my page, navigates to a different page then uses their back button to return to the original page. When I alert the referrer sometimes its blank and sometimes it has the address. I can't seem to find any pattern with when it is blank and when it is not. Does anyone know why it might be blank sometimes and others not? This is happening in IE7 and firefox over HTTP

Answer 1

This is most probably a bug in IE: http://webbugtrack.blogspot.com/2008/11/bug-421-ie-fails-to-pass-http-referer.html