jqery.height() returning no value in IE

jqery.height() returning no value in IE

I am trying to use jquery's .height function to dynamically size a <div> so that it sits just above the bottom edge of the user's screen, and taking into account 210px of padding from a banner at the top of the page. I am currently using this code:
<style>
        .cart{
                background:#66FFFF;
                position:fixed;
                width:20%;
                height:100%;
                right:10px;
                top:190px;
        }
</style>

<script type="text/javascript">
var cart_height = $("#cart").height() - 210;
$("#cart").height(cart_height);
</script>




This code works perfectly on firefox, however IE7 completely ignores it and sizes the box at 100%. When I print out the results from $("#cart").height() in IE7 it returns 0? Is this a known bug?