[Zope] #tree tag problem

I have a DTML Method "test" at .../foo/test. It contains a tree tag. Clicking on the icons to expand or contract the tree, I end up at .../foo/test/test?tree-c=.... This is OK, though it only works by accident of acquisition, but if I rename test to index_html, it doesn't work (.../foo/index_html/index_html?... ends up trying to get a docstring on None, for some reason). Is this a bug, or am I doing something wrong?