[Zope] ZCatalog range searches with floats

I'd like a quick piece of Catalog advice before I allow myself to get sucked into it with no chance of escape. I've got a few objects in the catalog, which have an attribute which may be a float, or may be blank. I want to do the equivalent of: "select * from catalog where attr > 4.53" I'm getting back *everything* in the catalog, whatever the value of the attribute. Before I dive in and work out exactly what's going on, I wanted to make sure this kind of thing is currently possible - every example of using the 'range' modifier currently uses a DateTime object for comparison. seb