IE Clientheight Problem

I'm trying to dynamically change the height of a div, and I have it
working in Firefox, but IE isn't letting me get the clientHeight of the
div. When I do an alert to check the clientHeight of the div, it tells
me 0.

Here is the function (at least the part that tries to get the height):

function resizeDivs(divnum){
var branddivid = 'branddiv' + divnum;
var branddiv = document.getElementById(branddivid);
var divheight = branddiv.clientHeight;
alert(divheight);
}