presence.py: 'NoneType' object has no attribute 'addCallbacks' #1

I'm periodically seeing exceptions.AttributeError: 'NoneType' object has no attribute 'addCallbacks' errors in the logs of the reference homeserver.

There does not appear to be a common type or timing of log entry immediately preceding the error.

Log extracts as follows.

Server started

2014-08-18 01:22:19,478 - twisted - 455 - INFO - Site starting on 8448

start_polling_presence

6 instances of the error relating to "/home/matrix/synapse/synapse/handlers/presence.py", line 426, in start_polling_presence:

• 2014-08-18 01:22:23,201 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 01:55:56,586 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 02:55:57,690 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 04:55:00,809 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 06:55:25,996 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 08:55:00,928 - twisted - 455 - ERROR - Unhandled error in Deferred:

2014-08-18 01:22:23,201 - twisted - 455 - ERROR - Unhandled error in Deferred:
2014-08-18 01:22:23,202 - twisted - 455 - ERROR - Unhandled Error
Traceback (most recent call last):
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 382, in callback
self._startRunCallbacks(result)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 490, in _startRunCallbacks
self._runCallbacks()
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 577, in _runCallbacks
current.result = callback(current.result, _args, *_kw)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 1155, in gotResult
_inlineCallbacks(r, g, deferred)
--- ---
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 1099, in _inlineCallbacks
result = g.send(result)
File "/home/matrix/synapse/synapse/handlers/presence.py", line 426, in start_polling_presence
yield defer.DeferredList(deferreds)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 820, in init
deferred.addCallbacks(self._cbDeferred, self._cbDeferred,
exceptions.AttributeError: 'NoneType' object has no attribute 'addCallbacks'

stop_polling_presence

6 instances of the error relating to "/home/matrix/synapse/synapse/handlers/presence.py", line 481, in stop_polling_presence:

• 2014-08-18 01:24:15,247 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 01:56:47,801 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 02:56:48,862 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 04:55:52,694 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 06:56:07,297 - twisted - 455 - ERROR - Unhandled error in Deferred:
• 2014-08-18 08:55:53,604 - twisted - 455 - ERROR - Unhandled error in Deferred:

2014-08-18 01:24:15,247 - twisted - 455 - ERROR - Unhandled error in Deferred:
2014-08-18 01:24:15,247 - twisted - 455 - ERROR - Unhandled Error
Traceback (most recent call last):
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 382, in callback
self._startRunCallbacks(result)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 490, in _startRunCallbacks
self._runCallbacks()
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 577, in _runCallbacks
current.result = callback(current.result, _args, *_kw)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 1155, in gotResult
_inlineCallbacks(r, g, deferred)
--- ---
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 1099, in _inlineCallbacks
result = g.send(result)
File "/home/matrix/synapse/synapse/handlers/presence.py", line 230, in set_state
self.stop_polling_presence(target_user)
File "/home/matrix/synapse/synapse/handlers/presence.py", line 481, in stop_polling_presence
return defer.DeferredList(deferreds)
File "/home/matrix/.local/lib/python2.7/site-packages/Twisted-14.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 820, in init
deferred.addCallbacks(self._cbDeferred, self._cbDeferred,
exceptions.AttributeError: 'NoneType' object has no attribute 'addCallbacks'