Without at least one worker process, things break:

Traceback (most recent call last):
  File "/home/ams/extern/ansible/ansible/lib/ansible/executor/process/result.py", line 103, in run
    result = self._read_worker_result()
  File "/home/ams/extern/ansible/ansible/lib/ansible/executor/process/result.py", line 69, in _read_worker_result
    (worker_prc, main_q, rslt_q) = self._workers[self._cur_worker]
IndexError: list index out of range