Invalid argument error

I’m getting the following error on a site I’m helping to develop:

Warning: Invalid argument supplied for foreach() in ../page_navmenu.php on line 24.

The plugin actually works on the top nav page using the menu I’ve assigned to the Page Navigation Menu plugin but not on sub pages. I’ve tried replacing that php file with a fresh downloaded one but it hasn’t helped.

The site is http://s134821.gridserver.com/about/history/

Thanks.

http://wordpress.org/extend/plugins/page-navigation-menu/