[Twisted-Python] Problem getting one woven.Widget to include another

[Alex]

I'm setting up a site using Woven, and I've hit a strange problem.

I've got a subclass of woven.widget.Widget called VolumeWidget, and that 
widget is used to display a list of links within a page.  I've got another 
class called VolumeListWidget, the only purpose of which is to display a 
list of these VolumeWidgets.

class VolumeListWidget(widgets.Widget):
     def setUp(self, request, node, data):
         for volume in data.get('volumes') or []:
             volume_model = VolumeModel(volume)
             volume_div = VolumeWidget(volume_model)
             volume_div.tagName = 'div'
             self.add(volume_div)

VolumeListWidget's not working.  Instead of displaying a <div> that 
contains all the stuff VolumeWidget is supposed to contain [as defined in 
VolumeWidget's setUp() function], it's displaying an empty <div/>.  I'm not 
sure what I'm doing wrong, if anything, but it's too strange to tell.

Example (along with some code) is available here:

http://polynode.com:8889/content.rpy/testvolumelist

To see a working example of VolumeWidget, go here:

http://polynode.com:8889/content.rpy/testvolume

I will appreciate any help anyone can lend me.