Description of problem:
- Create an alembic project
- Create a new revision
- Upgrade to this new revision
- Try to generate the sql to downgrade : $ alembic downgrade 3f04505c8221 --sql
- crash :)

Version-Release number of selected component:
python-alembic-0.3.4-4.fc18

Additional info:
cmdline:        /usr/bin/python /usr/bin/alembic downgrade 3f04505c8221 --sql
executable:     /usr/bin/alembic
kernel:         3.7.5-201.fc18.x86_64
uid:            1000

Truncated backtrace:
script.py:176:_downgrade_revs:AttributeError: 'NoneType' object has no attribute 'module'

Traceback (most recent call last):
  File "/usr/bin/alembic", line 9, in <module>
    load_entry_point('alembic==0.3.4', 'console_scripts', 'alembic')()
  File "/usr/lib/python2.7/site-packages/alembic/config.py", line 229, in main
    **dict((k, getattr(options, k)) for k in kwarg)
  File "/usr/lib/python2.7/site-packages/alembic/command.py", line 145, in downgrade
    script.run_env()
  File "/usr/lib/python2.7/site-packages/alembic/script.py", line 188, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/usr/lib/python2.7/site-packages/alembic/util.py", line 185, in load_python_file
    module = imp.load_source(module_id, path, open(path, 'rb'))
  File "alembic/env.py", line 68, in <module>
    run_migrations_offline()
  File "alembic/env.py", line 41, in run_migrations_offline
    context.run_migrations()
  File "<string>", line 7, in run_migrations
  File "/usr/lib/python2.7/site-packages/alembic/environment.py", line 450, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/usr/lib/python2.7/site-packages/alembic/migration.py", line 200, in run_migrations
    self):
  File "/usr/lib/python2.7/site-packages/alembic/command.py", line 134, in downgrade
    return script._downgrade_revs(revision, rev)
  File "/usr/lib/python2.7/site-packages/alembic/script.py", line 176, in _downgrade_revs
    for script in revs
AttributeError: 'NoneType' object has no attribute 'module'

Local variables in innermost frame:
revs: <generator object _iterate_revisions at 0x24882d0>
destination: '3f04505c8221'
self: <alembic.script.ScriptDirectory object at 0x23f1d50>
current_rev: None
script: None