Keep position fixed through absolute position failed in IE6

Question

I'm using javascript to compute bottom property to keep element like fixed position in IE6, here is my code:

var target = document.getElementsByTagName("div")[0];
var targetBottom = 200;

window.onscroll = function (e) {
    scrollTop = document.documentElement.scrollTop;
    target.style.bottom = targetBottom - scrollTop + "px";
}

However, this could't work in IE6, but in IE7 and above. So what's wrong with my code? How can I make it work in IE6?