TypeError: 'NoneType' object does not support item assignment

This error occurs at installing openstack with DevStack script. The error messages are repeated for a couple of times at very beginning of Swift console.

==============Swift Log=================
...
swift Calling S3Token middleware.
swift No Authorization header. skipping.
swift Using identity: {'roles': [u'admin'], 'user': u'glance', 'tenant': (u'922e234296a64e059b533aa14401ddf5', u'service')}
swift allow user with role admin as account admin (txn: tx8717265f001945da91acde230e06b400) (client_ip: 192.168.156.204)
swift ERROR Unhandled exception in request:
Traceback (most recent call last):
  File "/opt/stack/swift/swift/proxy/server.py", line 2032, in handle_request
    return handler(req)
  File "/opt/stack/swift/swift/common/utils.py", line 1262, in wrapped
    return func(*a, **kw)
  File "/opt/stack/swift/swift/proxy/server.py", line 101, in wrapped
    return func(*a, **kw)
  File "/opt/stack/swift/swift/proxy/server.py", line 1618, in HEAD
    return self.GETorHEAD(req, stats_type='HEAD')
  File "/opt/stack/swift/swift/proxy/server.py", line 1579, in GETorHEAD
    req.path_info, len(nodes))
  File "/opt/stack/swift/swift/proxy/server.py", line 825, in GETorHEAD_base
    source.getheader('x-timestamp')
TypeError: 'NoneType' object does not support item assignment (txn: tx8717265f001945da91acde230e06b400) (client_ip: 192.168.156.204)
....
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