[Zope] prefix attribute in dtml-in

Using Zope 2.8, I have two nested <dtml-in> tags that each loop through the results of different SQL methods. I'm trying to use the prefix attribute to differentiate the two, but I can't get it to work. Does anyone have any thoughts? <dtml-in expr="getConsulting()" prefix="topic"> <fieldset> <legend><dtml-var expr="sequence_var_fulltitle"> Sub Topics</legend> <dtml-in expr="getConsultingSub()" prefix="subtopic"> <fieldset> <legend>Topic <dtml-var subtopic_displayorder>: <dtml-var subtopic_menutitle></legend> </fieldset> </dtml-in> </fieldset> </dtml-in>