When wrong API key used, "AttributeError: 'str' object has no attribute 'get'" (on string object: data)

[JamesParrott]

data is set here:
self._data = response.json()
but requests' Response.json() could "return a dictionary, list, etc. depending on what is in the response.":

https://requests.readthedocs.io/en/latest/api/#requests.Response.json

Traceback (most recent call last):

  File "C:\...\core.py", line 302, in _send_email_to_all_dependents
    response = pystmark.send_batch([pystmark.Message(**payload)
                                    for payload in payloads
                                   ],
                                   api_key=os.getenv('POSTMARK_API_KEY')
                                  )
  File "C:\...\Lib\site-packages\pystmark.py", line 138, in send_batch
    return _default_pyst_batch_sender.send(messages=messages, api_key=api_key,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                           secure=secure, test=test,
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^
                                           **request_args)
                                           ^^^^^^^^^^^^^^^
  File "C:\...\Lib\site-packages\pystmark.py", line 1210, in send
    return super(BatchSender, self).send(message=messages, test=test,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                         api_key=api_key, secure=secure,
                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                         **request_args)
                                         ^^^^^^^^^^^^^^^
  File "C:\...\Lib\site-packages\pystmark.py", line 1124, in send
    return self._request(url, data=data, headers=headers, **request_args)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\...\Lib\site-packages\pystmark.py", line 1009, in _request
    return self.response_class(response, sender=self)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\...\Lib\site-packages\pystmark.py", line 820, in __init__
    self.messages = [MessageConfirmation(msg) for msg in data]
                     ~~~~~~~~~~~~~~~~~~~^^^^^
  File "C:\...\Lib\site-packages\pystmark.py", line 701, in __init__
    self.error_code = data.get('ErrorCode', 0)
                      ^^^^^^^^
AttributeError: 'str' object has no attribute 'get'