[py3] bug.subscribe() crashes with simplejson.scanner.JSONDecodeError

While porting Apport to python 3 I stumbled over this exception:

bzr branch lp:apport
cd apport
ln -s ../backends/packaging-apt-dpkg.py apport/packaging_impl.py
# work around bug 1425575
wget -O- http://people.canonical.com/~pitti/tmp/apport.workaround-1425575.patch | patch -p1
PYTHONPATH=. python3 apport/crashdb_impl/launchpad.py -v _T.test_2_update_traces

crashes with

ERROR: test_2_update_traces (__main__._T)
update_traces()
----------------------------------------------------------------------
Traceback (most recent call last):
  File "apport/crashdb_impl/launchpad.py", line 1329, in test_2_update_traces
    self.crashdb.update_traces(self.get_segv_report(), r, 'good retrace!')
  File "apport/crashdb_impl/launchpad.py", line 471, in update_traces
    self._subscribe_triaging_team(bug, report)
  File "apport/crashdb_impl/launchpad.py", line 930, in _subscribe_triaging_team
    bug.subscribe(person=person)
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 623, in __call__
    self.resource.lp_refresh()
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 760, in lp_refresh
    super(Entry, self).lp_refresh(new_url, etag)
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 347, in lp_refresh
    representation, self.JSON_MEDIA_TYPE)
  File "/usr/lib/python3/dist-packages/wadllib/application.py", line 363, in bind
    representation_definition)
  File "/usr/lib/python3/dist-packages/wadllib/application.py", line 300, in __init__
    _make_unicode(representation))
  File "/usr/lib/python3/dist-packages/simplejson/__init__.py", line 505, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3/dist-packages/simplejson/decoder.py", line 370, in decode
    obj, end = self.raw_decode(s)
  File "/usr/lib/python3/dist-packages/simplejson/decoder.py", line 400, in raw_decode
    return self.scan_once(s, idx=_w(s, idx).end())
simplejson.scanner.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Unfortunately I wasn't able to come up with a simple reproducer script for this. Something like

bug = lp.bugs[123456]
person = '%s~ubuntu-crashes-universe' % lp._root_uri
bug.subscribe(person=person)

works fine, so there must be some more internal state keeping which messes this up. Please let me know if you need a simpler reproducer for this, then I'll try harder.