ERROR:root:'NoneType' object is not iterable
Traceback (most recent call last):
  File "/home/antoine/.virtualenvs/swh/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/antoine/.virtualenvs/swh/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "</home/antoine/.virtualenvs/swh/lib/python3.7/site-packages/decorator.py:decorator-gen-44>", line 2, in revision_get
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/negotiation.py", line 148, in _negotiate
    return f.negotiator(*args, **kwargs)
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/negotiation.py", line 91, in __call__
    return formatter(result)
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/negotiation.py", line 64, in __call__
    self.render(obj), content_type=self.response_mimetype
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/__init__.py", line 87, in render
    return msgpack_dumps(obj, extra_encoders=self.extra_encoders)
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/serializers.py", line 227, in msgpack_dumps
    return msgpack.packb(data, use_bin_type=True, default=encode_types)
  File "/home/antoine/.virtualenvs/swh/lib/python3.7/site-packages/msgpack/__init__.py", line 46, in packb
    return Packer(**kwargs).pack(o)
  File "msgpack/_packer.pyx", line 284, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 290, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 287, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 277, in msgpack._cmsgpack.Packer._pack
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/api/serializers.py", line 217, in encode_types
    return list(obj)
  File "/home/antoine/swh/swh-environment/swh-core/swh/core/db/common.py", line 97, in _meth
    yield from meth(self, *args, db=db, cur=cur, **kwargs)
  File "/home/antoine/swh/swh-environment/swh-storage/swh/storage/storage.py", line 612, in revision_get
    data = converters.db_to_revision(dict(zip(db.revision_get_cols, line)))
  File "/home/antoine/swh/swh-environment/swh-storage/swh/storage/converters.py", line 237, in db_to_revision
    parents=tuple(parents),
  File "<attrs generated init swh.model.model.Revision>", line 14, in __init__
    _inst_dict['extra_headers'] = __attr_converter_extra_headers(extra_headers)
  File "/home/antoine/swh/swh-environment/swh-model/swh/model/model.py", line 453, in tuplify_extra_headers
    return tuple((k, v) for k, v in value)
TypeError: 'NoneType' object is not iterable