is_category in custom function will not display on site

I’ve written a custom function (in the Thesis theme) that will show a special ad on category pages that match a specific category. I think I have the syntax right but I can’t quite seem to get it to show.

Here’s the code:

if(is_category('36')) { ?>
		<div class="post_ad">
		<h3>Headline</h3>
		<p>Ad copy</p>
	</div>
	<?php }

I’ve tried using the category slug, the category name, and the category ID. I’ve even tried using in_category instead of is_category, but no matter what I do, this simply will not display.

What am I doing wrong?