Crash in update_state

I found this in my debug log, not sure what caused it:

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 "/usr/lib/python2.7/dist-packages/juju/state/service.py", line 494, in watcher
    yield callback(change_event)
  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/python2.7/dist-packages/juju/agents/unit.py", line 203, in cb_watch_config_changed
    yield self.workflow.fire_transition("reconfigure")
  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/python2.7/dist-packages/juju/lib/statemachine.py", line 190, in fire_transition
    yield self.set_state(transition.destination, **state_variables)
  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/python2.7/dist-packages/juju/lib/statemachine.py", line 233, in set_state
    yield self._store(dict(state=state, state_variables=variables))
  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/python2.7/dist-packages/juju/unit/workflow.py", line 239, in _store
    yield retry_change(self._client, self.zk_state_path, update_state)
  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/python2.7/dist-packages/txzookeeper/utils.py", line 54, in retry_change
    new_content = yield change_function(content, stat)
  File "/usr/lib/python2.7/dist-packages/juju/unit/workflow.py", line 231, in update_state
    unit_data = yaml.load(content)
  File "/usr/lib/python2.7/dist-packages/yaml/__init__.py", line 69, in load
    loader = Loader(stream)
  File "/usr/lib/python2.7/dist-packages/yaml/loader.py", line 34, in __init__
    Reader.__init__(self, stream)
  File "/usr/lib/python2.7/dist-packages/yaml/reader.py", line 85, in __init__
    self.determine_encoding()
  File "/usr/lib/python2.7/dist-packages/yaml/reader.py", line 124, in determine_encoding
    self.update_raw()
  File "/usr/lib/python2.7/dist-packages/yaml/reader.py", line 178, in update_raw
    data = self.stream.read(size)
exceptions.AttributeError: 'NoneType' object has no attribute 'read'