use_mirrors broken for DescriptorDownloader

When using the use_mirrors option for DescriptorDownloader, I get this:

Traceback (most recent call last):
  File "bushel.py", line 21, in <module>
    for desc in downloader.get_consensus().run():
  File "/usr/lib/python3/dist-packages/stem/descriptor/remote.py", line 723, in get_consensus
    consensus_query = self.query(resource, **query_args)
  File "/usr/lib/python3/dist-packages/stem/descriptor/remote.py", line 811, in query
    return Query(resource, **args)
  File "/usr/lib/python3/dist-packages/stem/descriptor/remote.py", line 375, in __init__
    self.endpoints.append(stem.DirPort(endpoint[0], endpoint[1]))  # TODO: remove this in stem 2.0
  File "/usr/lib/python3/dist-packages/stem/__init__.py", line 571, in __init__
    raise ValueError("'%s' isn't a valid port" % port)
ValueError: 'None' isn't a valid port

This is potentially due to relays that don't have dir ports, but this is just speculation.