Menu won't expands

D6.13, only standard Menu module activated, no additional menu-changing modules is used.

Way to reproduce a bug:
1. Create node, book or page type. Let it be Node 13.
2. Put link to the Node 13 into the menu (node/13).
3. Create at least one sub-menu item into the Node 13, like this:

• Node 13
  • Node 14

4. Click on Node 13 menu item, Node 14 sub-menu will expand.

Bug starts here
5. Change menu item for Node 13 to Booktree link to Node 13 (so replace node/13 with booktree/13).
6. Click on changed menu item, sub-menu with Node 14 will not appear, so menu leafs will not expands.

If you need any additional information, please ask.