wp_nav_menu() 'menu_class' usage? (bug?)

Hello,
I’m trying to set the class on the ul element generated by the wp_nav_menu function.

I’m using this php code:
<?php wp_nav_menu(array('menu_class' => 'menuclass', 'container_class' => 'containerclass')); ?>

I’d expect it to apply ‘menuclass’ to the ul, and ‘containerclass’ to the container div… But it doesn’t. It prints like this:

<div class="menuclass">
<ul>
<li class="current_page_item"><a title="Home" href="#">Page 1</a></li>
<li><a href="#">Page 2</a>
</ul>
</div>

Is this a bug in the way the function works, or is the documentation incorrect?