Mock call to subprocess.Popen failing with StopIteration

[tharpa]

A unit test that I have inherited is now failing on

p = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE)
logging.warning('Not hitting here.')

with a traceback of

Error:
File "/usr1/tsmith/miniconda3/lib/python3.7/unittest/mock.py", line 960, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr1/tsmith/miniconda3/lib/python3.7/unittest/mock.py", line 1022, in _mock_call
    result = next(effect)

StopIteration`

Any idea how to investigate this?