Request.UrlReferrer returning null with IE 8

问题:

I'm trying to access the URL in Request.UrlReferrer. It works fine in Firefox and returns me the correct value. But, when I try and access it when debugging from Internet Explorer, it returns null.

My code:

if (Request.UrlReferrer != null)
{
    if (Request.UrlReferrer.ToString().IndexOf("AspxPage.aspx") > -1)
    {
        // ------
    }
}

Why is this happening?