Thresher is not filtering IP addresses correctly

[krmaxwell]

(venv)kmaxwell@newton:~/src/combine$ python winnow.py
Traceback (most recent call last):
  File "winnow.py", line 88, in <module>
    winnow('crop.json', 'winnowed.json')
  File "winnow.py", line 78, in winnow
    ipaddr = IPAddress(addr)
  File "/home/kmaxwell/src/combine/venv/local/lib/python2.7/site-packages/netaddr/ip/__init__.py", line 307, in __init__
    'address from %r' % addr)
netaddr.core.AddrFormatError: failed to detect a valid IP address from u'199.222.35.192.in-addr.arpa.'