Custom menu missing on search page.

I’ve been scratching my head over this one, so thanks in advance for any help. Much appreciated.

I’ve got a menu in WP 3.0.1 which I call in header.php using:

wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary' ) );

It’s showing up fine on every page **apart** from search results. The search.php file starts with a standard get_header(); so it must be the wp_nav_menu code right? But nothing’s wrapped in an is_search() conditional or anything.

The output HTML should look like this:

<div class="menu-header">
<ul id="menu-main-menu" class="menu">
<li class="menu-item">
<a>Link 1</a>
</li>
<li class="menu-item">
<a>Link 2</a>
</li>
</div>

But instead, it gets as far as the <ul> and doesn’t output any <li>s:

<div class="menu-header">
<ul id="menu-main-menu" class="menu">
</div>

Most strange. Has anyone else come across this before? Is it just far too late and I’m missing something obvious?