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something fishy with pyccn, it happens at the end of the process

Traceback (most recent call last):
   File "./ndn-ls-keys.py", line 185, in upcall
     if kind == pyccn.UPCALL_CONTENT_UNVERIFIED:
 AttributeError: 'NoneType' object has no attribute 'UPCALL_CONTENT_UNVERIFIED'

No idea why pyccn is treated as a 'NoneType' object