Jquery.unload problem in IE

I have the following code that works flawlessly in FF, but in IE I get the error "Invalid property value"

The code in the questions looks like this:

pageOpen = new Date();

dropper = new Image();

$(window).unload(function() {

  pageClose = new Date();

  sionara = (pageOpen.getTime() - pageClose.getTime())/-1000;

  dropper.src = "http://redacted.domain.com/1/%%PAGE%%/1%%REALRAND%%@x80&XE&ABC_TimeSpent="+sionara+"_seconds&XE";

  alert("close " + sionara);

});