Help - title by another field, then got error when deleting object

we have a content type,
where must set the title according to another field of content type(ct),
here is the code for ct:

def title(self):
    return self.dept_name() # from oracle... quick query, time-consuming?

def title(self, value):

everything works fine even with portal_catalog,
but when we try to delete the ct from Plone menu (Actions->Delete),
we got such few weird index errorss:

[Zope.KeywordIndex:138][waitress-0] KeywordIndex: Attempt to unindex nonexistent document with id -1707800307
/eggs/Products.ZCatalog-5.4-py3.8.egg/Products/PluginIndexes/KeywordIndex/", line 136, in unindex_object
del self._unindex[documentId]
KeyError: -1707800307

the ct is cointainer, not document.

even after brutal Clear and Rebuild catalog provided we still see same errros.
obj.reindexObject() does not help.

Please could you give any idea? Many thanks for your time.