Cannot run migration when using timestamp version numbers

The changeset method in migrate/versioning/repository.py (around line 200), does not work for timestamped version numbers. The list comprehension which determines the set of versions between :start: and :end: assumes that the versions are 1 apart from each other. This is obviously wrong (most of the time) when using timestamped version numbers, resulting in the following traceback:

Traceback (most recent call last):
  File "/Users/andrew/work/incubator/hilo-backend/venv/bin/migrate", line 11, in <module>
    sys.exit(main())
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/shell.py", line 209, in main
    ret = command_func(**kwargs)
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/api.py", line 186, in upgrade
    return _migrate(url, repository, version, upgrade=True, err=err, **opts)
  File "<decorator-gen-15>", line 2, in _migrate
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/util/__init__.py", line 160, in with_engine
    return f(*a, **kw)
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/api.py", line 345, in _migrate
    changeset = schema.changeset(version)
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/schema.py", line 82, in changeset
    changeset = self.repository.changeset(database, start_ver, version)
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/repository.py", line 225, in changeset
    changes = [self.version(v).script(database, op) for v in versions]
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/repository.py", line 225, in <listcomp>
    changes = [self.version(v).script(database, op) for v in versions]
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/repository.py", line 189, in version
    return self.versions.version(*p, **k)
  File "/Users/andrew/work/incubator/hilo-backend/venv/lib/python3.4/site-packages/migrate/versioning/version.py", line 163, in version
    return self.versions[VerNum(vernum)]
KeyError: <VerNum(1)>