terminate_instances raises NoneType not iterable on machine shutdown with Openstack

2011-09-29 10:24:27,572 provision:ec2: juju.agents.provision DEBUG: Machines changed old:set([0, 1, 2, 3, 4]) new:set([0, 1, 2, 3])
2011-09-29 10:24:27,708 provision:ec2: juju.agents.provision INFO: Shutting down machine id:i-000005fa ...
2011-09-29 10:24:27,892 provision:ec2: twisted ERROR: Unhandled error in Deferred:
2011-09-29 10:24:27,895 provision:ec2: twisted ERROR: Unhandled Error
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 388, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 455, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 542, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1076, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1018, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 350, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "juju/state/base.py", line 138, in __topology_changed
    yield watch_topology_function(self._old_topology, new_topology)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1018, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 350, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/juju/juju/juju/agents/provision.py", line 206, in process_machines
    yield self.provider.shutdown_machine(machine)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1018, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 350, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "juju/providers/ec2/__init__.py", line 168, in shutdown_machines
    terminated = yield self.ec2.terminate_instances(*killable_ids)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 542, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/txaws/ec2/client.py", line 633, in terminate_instances
    for instance in root.find("instancesSet"):
exceptions.TypeError: 'NoneType' object is not iterable

This only happens when talking to openstack, not EC2. It does not cause any failures, just an unexpected error message.