[Zope] Why is index_html() called instead of __call__()?

I've been trying to use PropertyFolder for awhile now. Almost all of my projects depend heavily on functionality that it would make *much* cleaner. I'm running a fairly standard Debian distribution of Zope, but I have to modify PropertyFolder to define index_html() instead of __call__(), otherwise the "object_type" renderer is ignored. I'm not sure PropertyFolder is to blame. Should an object's __call__() method be used to render it? Any ideas why index_html() might be used instead? Where could I go to track down the misbehavior? I'd like to fix this so I can move on to some enhancements. Thank you. --kyler