Notice Message when dispaying a Book

When the page was:
http://www.mysite.com/?q=booktree/15

I got the following message:
Notice: Undefined index: fr in booktree_indice() (line 98 of /homez.462/abdevnet/www/contractdb-project/sites/all/modules/booktree/booktree.module).

The book was displayed, but there no tabs for the second and others pages of the book. All pages are aligned on the left.

Thanks