TypeError: AES.__init__() argument 1 must be string or read-only character buffer, not None

[zooko]

In comment:17:ticket:1939 there was this stack trace from Tahoe-LAFS 1.9.2 during a mutable mapupdate:

2013-04-02_00:00:45.292535Z [190484]: WEIRD {'f_value':'AES.__init__() argument 1 must be string or read-only character buffer, not None', 'facility':'tahoe.mutable.mapupdate', 'failure':<foolscap.call.CopiedFailure exceptions.TypeError>, 'format':'error during privkey query: %(f_value)s', 'incarnation':('\xd0N\xca\xf7\xa7l\x8f\xce', None), 'parent':190483, ...} FAILURE:
[CopiedFailure instance: Traceback from remote host -- Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 551, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/pymodules/python2.7/allmydata/mutable/servermap.py", line 723, in _got_results
    d4.addCallback(lambda results, shnum=shnum:
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 301, in addCallback
    callbackKeywords=kw)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 290, in addCallbacks
    self._runCallbacks()
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 551, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/pymodules/python2.7/allmydata/mutable/servermap.py", line 724, in <lambda>
    self._try_to_validate_privkey(results, server, shnum, lp))
  File "/usr/lib/pymodules/python2.7/allmydata/mutable/servermap.py", line 929, in _try_to_validate_privkey
    alleged_privkey_s = self._node._decrypt_privkey(enc_privkey)
  File "/usr/lib/pymodules/python2.7/allmydata/mutable/filenode.py", line 168, in _decrypt_privkey
    enc = AES(self._writekey)
exceptions.TypeError: AES.__init__() argument 1 must be string or read-only character buffer, not None
]