[Zope] Behaviour of ZCatalog changed?

Hi, Just a quick question that has been bothering me for a while now: Has the behaviour of ZCatalog changed recently? In earlier versions I did a query like this: <dtml-in "Catalog(meta_type='Track', track_active='')"> This query gave me all objects of type 'Track', which had a blank track_active property. The ones which had 'on' in track_active were not displayed. When I do this query now, it gives me both the tracks that has 'on' in the track_active property, and those that don't. I've tried track_active='None' and various other ways, but I can't get it working. What I need is a way to display only the ones with an empty value in track_active. Could someone enlighten me? -- Alexander (I'm running Zope 2.3.2, by the way)