IE 5.5 Not Playing Ball?

i've got the following code that checks the browser width and then redirect is it's under 500px, but IE 5.5 just redirects regardless:

<script language="javascript" type="text/javascript">
function checkBrowserWidth()
{
var theWidth = getBrowserWidth();
if (theWidth < 499)
{
window.location.replace("/handheld/");
}
return true;
}
function getBrowserWidth()
{
if (window.innerWidth)
{
return window.innerWidth;
}
else if (document.documentElement && document.documentElement.clientWidth != 0)
{
return document.documentElement.clientWidth;
}
else if (document.body)
{
return document.body.clientWidth;
}
return 0;
}
checkBrowserWidth();
window.onload = checkBrowserWidth;
</script>

any ideas??