Error when using scrollHeight to determine if at bottom of a div

Question

Found a great tip from Ca Phun Ung about determining if I'm at the bottom of a div:

http://yelotofu.com/2008/10/jquery-how-to-tell-if-youre-scroll-to-bottom/

var elem = $('#box');
if (elem[0].scrollHeight - elem.scrollTop() == elem.outerHeight()) {
  // We're at the bottom.
}

Works well in FF and Safari, of course, but I have one error I'm getting in IE7:

Error:'0.scrollHeight' is null or not an object.

Any idea what might be causing that?