Add a class to menu ul tag?

I use this code for my menu:
<?php wp_nav_menu( array(‘menu’ => ‘header-menu’ )); ?>

How can I add a css class to the

I know that I can wrap it in another div, but my theme is coded in html/css, and I’m coverting it to wordpress, and right now, it’s <ul class=”head-menu”>, so I don’t want to edit the css files.

Thanx a bunch!