Editing layer hierarchy generates server error

Attempting to add additional layers to an existing document (empty) causes Server Error:

Server Error
An internal server error has occurred. This most likely means that you have found a bug in SchoolTool. Please report it on the issue tracker.

Exception
AttributeError: 'list' object has no attribute 'add'

Traceback
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 132, in publish
  result = publication.callObject(request, obj)
File "/usr/lib/python2.7/dist-packages/zope/app/publication/zopepublication.py", line 207, in callObject
  return mapply(ob, request.getPositionalArguments(), request)
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 107, in mapply
  return debug_call(obj, args)
Extra information: <schooltool.skin.flourish.zcml_content.AvailableLayersTable_available_table object at 0xb225efac>
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 113, in debug_call
  return obj(*args)
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/viewlet.py", line 253, in __call__
  return ViewletManager.__call__(self, *args, **kw)
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/content.py", line 58, in __call__
  self.update()
File "/usr/lib/python2.7/dist-packages/schooltool/app/browser/app.py", line 296, in update
  self.applyChanges()
File "/usr/lib/python2.7/dist-packages/schooltool/app/browser/app.py", line 308, in applyChanges
  self.submitItems()
File "/usr/lib/python2.7/dist-packages/schooltool/app/browser/app.py", line 350, in submitItems
  self.view.add(removeSecurityProxy(item))
File "/usr/lib/python2.7/dist-packages/schooltool/app/browser/app.py", line 439, in add
  collection.add(item)