Error with None result

[ooq]

def f_none(key):
    return None
f = pwex.call_async(f_none, None)
pywren.wait([f])
print(f.result())

The above code errs at:

     97         if self._state == JobState.success:
---> 98             assert self._return_val != None
     99             return self._return_val