GetElementById Return Value Differences In Firefox/IE

In Firefox 2, document.getElementById is returning an HTMLDivElement,
in IE 7, it is returning an Object.

For example:

<div id="errorTableDiv">
</div>
....

errorTableDiv = document.getElementById("errorTableDiv");
alert(errorTableDiv);
errorTableDiv.appendChild(someTableNode);

Firefox prints HTMLDivElement and lets me appendChild() later on
IE prints Object and gives an error on the appendChild()

I do not understand why this happens or what the most socially
acceptable way to fix it is.