Head First JavaScript - document.body.clientHeight not in intellisense

When I created the function resizeRock() and typed in the line
document.getElementById("rockImg").style.height = (document.body.clientHeight - 100) * 0.9;
the attribute clientHeight does not show up in intellisense and the height that is returned is 81, so the picture doesn't resize. If I change the 100 to 10, it will resize but I'm wondering if I need a Javascript update or if there is another problem. I am using Internet Explorer version 8.0.6001. Thanks.