[Zope] Problem with Topic Index

I have a trange problem with topic index (on Zope 2.6.1 running on Linux). I created a filtered set with the expression (o.title == 'aa'). I catalogued only one document (this document has title 'aa') and filtered set reports it has one document. Then I went to the Catalog of the ZCatalog, and I removed the catalogued object. All indexes removed the object except this filtered set which still contains one object (ZCatalog itself contains zero objects). And then querying topic index with this filtered set produces KeyError in ZCatalog. It seems like topic index is ignoring unindex operation. Is there anything special that needs to be done with topic indexes ? Vladimir