#!
$ python -m alembic.config upgrade head
INFO  [alembic.migration] Context impl PostgresqlImpl.
INFO  [alembic.migration] Will assume transactional DDL.
Traceback (most recent call last):
...
  File "alembic/migration.py", line 296, in run_migrations
    for step in self._migrations_fn(heads, self):
  File "alembic/command.py", line 154, in upgrade
    return script._upgrade_revs(revision, rev)
  File "alembic/script.py", line 303, in _upgrade_revs
    revs = list(revs)
  File "alembic/revision.py", line 581, in _iterate_revisions
    upper_ancestors = set(self._get_ancestor_nodes(uppers, check=True))
  File "alembic/revision.py", line 551, in _iterate_related_revisions
    map_[rev_id] for rev_id in fn(rev))
  File "alembic/revision.py", line 531, in <lambda>
    fn = lambda rev: rev._all_down_revisions
AttributeError: 'NoneType' object has no attribute '_all_down_revisions'
classics-MacBook-Pro-2:alembic classic$