[Zope] sort_on keyword in ZCatalog searches?

As per Zen's How-To: http://www.zope.org/Members/Zen/howto/AdvZCatalogSearching I have been trying to sort my ZCatalog searches on a FieldIndex: <dtml-in "publicationCatalog(Authors='Kivshar',sort_on='Year')"> <dtml-var Authors>, <dtml-var Journal>,<dtml-var Year></dtml-with><br> </dtml-in> On my 2.1.0 Zope, this search returns empty. On my 2.1.4 Zope, this search returns a list of the correct length but with duplicate entries corresponding to the first match. Year is definately af FieldIndex. Any ideas on the source of this problem? Cheers, Darran.