[Zope] Unexpected transation using Catalog

I have a Catalog which has indexed several properties, and a form where you can enter a property name and a property value. The catalog is searched for objects where the named property has that value. I am searching the Catalog using: <dtml-let content="Catalog.searchResults({index_property:index_value}) "> Unfortunately this search is causing extra transactions to appear in the undo log. This happens on the first search for each pair of parameters, after starting Zope or flushing the cache. Am I doing something wrong, or is this a bug? Thanks, Toby