generate_overall cause error when computing summary

[alexgoft]

I am trying to compute summary of several accumulators the following way:

mh = mm.metrics.create()

summary = mh.compute_many([all_0, all_1],
                          names=['acc_0', 'acc_1'],
                          metrics=METRICS_MOT,
                          generate_overall=True)

compute_many works without generate_overall, however when i pass it True, it fails:

  File "/hdd/venv/lib/python3.6/site-packages/motmetrics/metrics.py", line 328, in _compute_overall
    v = cache[depname] = self._compute_overall(partials, depname, cache, parent=name)
  File "/hdd/venv/lib/python3.6/site-packages/motmetrics/metrics.py", line 325, in _compute_overall
    for depname in minfo['deps_m']:
TypeError: 'NoneType' object is not iterable

EDIT:
example script fails as well:: when passing generate_overall=True in the last call to compute_many