booktree.module prints an "n/a" at the top when viewed.

Using booktree 6.x-1.4 multi-site setup on Drupal 6.14 with a custom theme.
I narrowed down the offending code to this line:

  drupal_set_title(check_plain($node->title));
>  $content = '<p>'. check_markup($node->body, $node->format, FALSE) .'</p>'; 
  $ricursione = 1;

The root node is set to 0 in order to display the contents of all the books' sub-trees in the site.
But on the first line when viewing mysite.org/site_name/booktree printed n/a
I changed the line to
$content = '<h1>'. variable_get('site_name') .'</h1>';
so it now displays the site name at the top. I'm not sure if the check_markup call was really important in certain situations, but it might be worth changing the module in future releases so it doesn't print out n/a.
My personal fix probably isn't an acceptable solution because some people want to display the content of specific books rather than the entire site's sub-trees.
Please comment why the call to check_markup was returning n/a if you have an idea on that, or challenge me to write up some conditionals to either print the site_name if the root node is 0 or the root node's title. Or if you have time, write it yourself and contrib! :)

Thanks,
Milo