AttributeError: 'NoneType' object has no attribute 'volume'

[gustavofonseca]

Traceback (most recent call last):
  File "/Users/gustavofonseca/prj/oai-pmh/tests/test_articlemeta.py", line 112, in test_get_known_resource_returns_namedtuple_resource
    self.assertIsInstance(am.get('validID'), entities.Resource)
  File "/Users/gustavofonseca/prj/oai-pmh/oaipmh/articlemeta.py", line 290, in get
    return ArticleResourceFacade(doc).to_resource()
  File "/Users/gustavofonseca/prj/oai-pmh/oaipmh/articlemeta.py", line 238, in to_resource
    source=self.source(),
  File "/Users/gustavofonseca/prj/oai-pmh/oaipmh/articlemeta.py", line 211, in source
    return [self.article.bibliographic_legends().get('descriptive_format')]
  File "/Users/gustavofonseca/prj/envs/oai-pmh/lib/python3.6/site-packages/xylose/scielodocument.py", line 1486, in bibliographic_legends
    self.issue.volume,
AttributeError: 'NoneType' object has no attribute 'volume'