I'd like my WordPress menu to highlight the category when a single post is displayed.

This example from the WordPress codex properly applies the class of "current" in category view, but does not apply the class (although it should) when switching to single-post view:

<li<?php if ( is_category('10') || in_category('10') && !is_single())
{ echo ' class="current"'; } ?>><a href="/?cat=10">My Category</a></li>


Any thoughts?

Thanks.