Pickling error whilst pickling mission instance

[ChrisTimperley]

Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/queues.py", line 234, in _feed
    obj = _ForkingPickler.dumps(obj)
  File "/usr/lib/python3.6/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
_pickle.PicklingError: Can't pickle <class 'houston.command.MAV_CMD_NAV_TAKEOFF'>: attribute lookup MAV_CMD_NAV_TAKEOFF on houston.command failed