not able/failing to stop app

successfully instantiating client and retrieving an app with
app = client.v2.apps.get_first(**{'name': <app_name>})

successfully checking app object with
type(app)
app.summary()

but getting this exception when running
app.stop()
would you please help? is there anything I maybe missing?

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/v2/apps.py", line 22, in stop
    return self.client.v2.apps.stop(self['metadata']['guid'])
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/v2/apps.py", line 89, in stop
    result = super(AppManager, self)._update(application_guid, dict(state='STOPPED'))
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/v2/entities.py", line 79, in _update
    return self._put(url, data, **kwargs)
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/v2/entities.py", line 91, in _put
    response = self.client.put(url, json=data, **kwargs)
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/client.py", line 175, in put
    return CloudFoundryClient._check_response(response)
  File "/Library/Python/2.7/site-packages/cloudfoundry_client/client.py", line 194, in _check_response
    raise InvalidStatusCode(response.status_code, body)
cloudfoundry_client.errors.InvalidStatusCode: 400 : {"code": 1001, "error_code": "CF-MessageParseError", "description": "Request invalid due to parse error: Decoded JSON cannot be nil"}

Beginne damit, app.stop() mit dem im Issue gezeigten Client-Setup zu reproduzieren, und verfolge anschließend die Anfrage durch cloudfoundry_client/v2/apps.py, v2/entities.py und client.py. Untersuche das vom PUT gesendete JSON und die resultierende Antwort; fertig ist die Aufgabe, wenn das Stoppen der abgerufenen App nicht mehr den 400 CF-MessageParseError erzeugt.