Downloading server descriptors crashes with a TypeError

The crash happens randomly, it must be tested multiple times until it appears.

from stem.descriptor.remote import DescriptorDownloader

downloader = DescriptorDownloader(use_mirrors=True, timeout=120, retries=1,
                                  block=True)
downloader.get_server_descriptors().run()

Traceback (most recent call last):
  File "E:\dev\prs\main\tortest.py", line 5, in <module>
    downloader.get_server_descriptors().run()
  File "e:\dev\envs\py34_x86\Python\lib\site-packages\stem\descriptor\remote.py", line 293, in run
    return list(self._run(suppress))
  File "e:\dev\envs\py34_x86\Python\lib\site-packages\stem\descriptor\remote.py", line 304, in _run
    raise self.error
  File "e:\dev\envs\py34_x86\Python\lib\site-packages\stem\descriptor\remote.py", line 357, in _download_descriptors
    self.download_url = self._pick_url(use_authority)
  File "e:\dev\envs\py34_x86\Python\lib\site-packages\stem\descriptor\remote.py", line 347, in _pick_url
    authority = random.choice(filter(HAS_V3IDENT, get_authorities().values()))
  File "e:\dev\envs\py34_x86\Python\lib\random.py", line 253, in choice
    i = self._randbelow(len(seq))
TypeError: object of type 'filter' has no len()

Trac:
Username: trodun