spurious failure of test_readv_with_adjust_for_latency(HttpTransport_urllib,HTTPSServer_urllib) on armhf

We're getting a spurious failure of one of the https tests on ARM:

ERROR: bzrlib.tests.per_transport.TransportTests.test_readv_with_adjust_for_latency(HttpTransport_urllib,HTTPSServer_urllib)
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_StringException: Text attachment: log
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Text attachment: traceback-1
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Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/testtools/runtest.py", line 169, in _run_user
    return fn(*args, **kwargs)
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/test_server.py", line 569, in stop_server
    self.server.stop_client_connections()
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/test_server.py", line 373, in stop_client_connections
    self.shutdown_client(c)
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/test_server.py", line 487, in shutdown_client
    connection_thread.join()
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/test_server.py", line 256, in join
    super(TestThread, self).join(timeout)
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/cethread.py", line 126, in run
    super(CatchingExceptionThread, self).run()
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/test_server.py", line 445, in process_request_thread
    self, request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 585, in process_request_thread
    self.handle_error(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 582, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 323, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.7/SocketServer.py", line 638, in __init__
    self.handle()
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/http_server.py", line 68, in handle
    SimpleHTTPServer.SimpleHTTPRequestHandler.handle(self)
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 345, in handle
    self.handle_one_request()
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/http_server.py", line 82, in handle_one_request
    self._handle_one_request()
  File "/build/buildd/bzr-2.5.0~beta4/build/lib.linux-armv7l-2.7/bzrlib/tests/http_server.py", line 129, in _handle_one_request
    SimpleHTTPServer.SimpleHTTPRequestHandler.handle_one_request(self)
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 313, in handle_one_request
    self.raw_requestline = self.rfile.readline(65537)
  File "/usr/lib/python2.7/socket.py", line 476, in readline
    data = self._sock.recv(self._rbufsize)
  File "/usr/lib/python2.7/ssl.py", line 232, in recv
    return self.read(buflen)
  File "/usr/lib/python2.7/ssl.py", line 151, in read
    return self._sslobj.read(len)
AttributeError: 'NoneType' object has no attribute 'read'