Undefined index: in menu_configure() (line 682 of menu_admin.inc)

Hello,

using drupal-7.0 with CentOS 5.5, PostgreSQL 8.4.7, PHP 5.3.

I've created a book as a FAQ for my newly installed site and added each page to the Main Menu. And I display Main Menu in the 1st footer column. Because I didn't want the tabs at the top to be shown as well, I went to the Home » Administration » Structure » Menus and selected the settings as displayed in the attached screenshot.

After I clicked Save button I've got the warning:

Notice: Undefined index: in menu_configure() (line 682 of /var/www/html/drupal/modules/menu/menu.admin.inc).

That line in menu.admin.inc is:

'#description' => t('Select the source for the Secondary links. An advanced option allows you to use the same source for both Main links (currently %main) and Secondary links: if your source menu has two levels of hierarchy, the top level menu links will appear in the Main links, and the children of the active link will appear in the Secondary links.', array('%main' => $menu_options[$main])),

Thanks
Alex