Custom menu on search page missing

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 开发者_StackOverflow社区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>
    </ul>
</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">
    </ul>
</div>

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

@ZoulRic's code works fine, but it breaks the search function in the Media Library. To prevent this add:

if( !is_admin() )

just before the last line, so your code look like:

function menu_fix_on_search_page( $query ) {
   if(is_search()){
       $query->set( 'post_type', array(
        'attachment', 'post', 'nav_menu_item', 'film', 'music'
          ));
         return $query;
         }
   }
if( !is_admin() ) add_filter( 'pre_get_posts', 'menu_fix_on_search_page' );

You may may have set a filter that is intercepting the query responsible for generating the navigation menu links, or more specifically, overriding the 'nav_menu_item' post type required by the query object.

Try this code:

function menu_fix_on_search_page( $query ) {
    if(is_search()){
        $query->set( 'post_type', array(
         'post', 'nav_menu_item'
            ));
          return $query;
    }
}
add_filter( 'pre_get_posts', 'menu_fix_on_search_page' );

Paste this into functions.php in your theme files.