HS Book implementation allows a book page to set itself as parent

It appears that my installation allows me to set a book page as it's own parent.

Doing so, and saving the node, results in a mysql memory error. I assume this is due to the book module trying to build the self-recursive page list. Future visits to the same node result in the same.

Steps to reproduce (assuming module installed):

1. Create new book page (don't use existing content, you may have to delete it to recover)
2. Edit book page
3. Set the node parent to itself in the hierarchal select box
4. Submit

Result: Mysql memory error any time that node is visited.