[Zope] Clear catalog problem

Hi all, In my Zope 2.5 root folder I have a vocabulary with id Vocabulary and a Zcatalog that uses (but doesn't contain) Vocabulary. If I use "Clear Catalog" in advanced tab, and then "Find and Catalog" in "Find Objects" tab, in Catalog I have the same object number I had before clear, but Vocabulary remains empty (If I clear a zcatalog containing vocabulary, with "find object" I have in vocabulary the same number of words I had before clear). Can anyone explain me this behaviour (I haven't found informations with Google)? Thanks Michele