为什么在IE 7.0中不行?

[LuckyBoy_Bob]

<div class="TreeMenu" id="CategoryTree" ></div>
<script type="text/javascript">
//设置高度
document.getElementById("CategoryTree").style.height = document.body.offsetHeight-message.offsetHeight;
.....
</script>

#CategoryTree
{
width:100%;
border:0px solid #99BEEF;
background:#D7EAF6;
color:inherit;
margin:0px;
padding:0px;
position:relative;
overflow:auto;
left:0px;
z-index:1;
}

为什么在IE 6.0中可以根据内容和屏幕自动设定流动条,而到了IE 7.0中,什么都显示不见了?