machines read API endpoint giving circular reference error

[Newell Jensen]

$ maas admin machines read
Circular reference detected while emitting response

Here is the stacktrace:

2017-04-19 22:10:32 regiond: [info] 127.0.0.1 GET /MAAS/api/2.0/machines/ HTTP/1.1 --> 500 INTERNAL_SERVER_ERROR (referrer: -; agent: Python-httplib2/0.9.1 (gzip))
2017-04-19 22:10:43 regiond: [info] 10.1.0.3 GET /MAAS/rpc/ HTTP/1.0 --> 200 OK (referrer: -; agent: provisioningserver.rpc.clusterservice.ClusterClientService)
2017-04-19 22:11:00 maasserver: [error] ################################ Exception: Circular reference detected while emitting response ################################
2017-04-19 22:11:00 maasserver: [error] Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 181, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 77, in __exit__
    self.gen.throw(type, value, traceback)
  File "/usr/lib/python3/dist-packages/maasserver/utils/async.py", line 195, in savepoint
    yield
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 181, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 59, in __call__
    response = upcall(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/decorators/vary.py", line 21, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/piston3/resource.py", line 229, in __call__
    else: stream = srl.render(request)
  File "/usr/lib/python3/dist-packages/piston3/emitters.py", line 411, in render
    seria = json.dumps(self.construct(), cls=DateTimeAwareJSONEncoder, ensure_ascii=False, indent=4)
  File "<string>", line 236, in emitter_new_construct
  File "<string>", line 26, in _any
  File "<string>", line 220, in _qs
  File "<string>", line 220, in <listcomp>
  File "<string>", line 34, in _any
  File "<string>", line 162, in _model
  File "<string>", line 28, in _any
  File "<string>", line 226, in _list
  File "<string>", line 226, in <listcomp>
  File "<string>", line 34, in _any
  File "<string>", line 192, in _model
  File "<string>", line 20, in _any
RuntimeError: Circular reference detected while emitting response