Menu not showing in theme preview

Hi,

I’m working on a new theme and have the following problem… When I view the theme in the front-end, everything shows up perfectly. However, when I view the theme in through theme preview option in the admin, the menu falls back to its backup form.

I’m using the following code to embed the menu:

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

Any ideas why this is happening? Thanks a lot!