no ranges for zoned indexterms?

When I define a ranged indexterm using startofrange and endofrange, like 
this

    <indexterm id="inputmasks2.i" class="startofrange">
        <primary>editboxes</primary>
        <secondary>input mask</secondary>
    </indexterm>
    ...
    <indexterm startref="inputmasks2.i" class="endofrange"/>

I get a range of page numbers in my resulting PDF index, like this:

    editboxes
      input mask, 111-115

but if I use a zoned indexterm, like this:

    <indexterm zone="editboxesetc.editbox>
        <primary>editboxes</primary>
        <secondary>input mask</secondary>
    </indexterm>

the resulting index has no range. It includes only the first page of the 
referenced
entity, like this:

    editboxes
      input mask, 111

Isn't it supposed to give me a range? Is this a bug in the FO 
stylesheets? Or perhaps am I doing
something stupid? (The entity does in fact span over the four pages. And 
a  quick look at the
generated FO leads me to believe that the problem is not in the 
subsequent processing of FO to
PDF.)

Dave Bridgeland