hmm_treebank_pos_tagger doesn't load

In [7]: _POS_TAGGER = 'taggers/hmm_treebank_pos_tagger/treebank.tagger.pickle.gz'

In [8]: pickle.load(nltk.data.find(_POS_TAGGER).open())
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-3e64e4dab40b> in <module>()
----> 1 pickle.load(nltk.data.find(_POS_TAGGER).open())

/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.pyc in load(file)
   1376 
   1377 def load(file):
-> 1378     return Unpickler(file).load()
   1379 
   1380 def loads(str):

/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.pyc in load(self)
    856             while 1:
    857                 key = read(1)
--> 858                 dispatch[key](self)
    859         except _Stop, stopinst:
    860             return stopinst.value

/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.pyc in load_reduce(self)
   1131         args = stack.pop()
   1132         func = stack[-1]
-> 1133         value = func(*args)
   1134         stack[-1] = value
   1135     dispatch[REDUCE] = load_reduce

/Users/kmike/envs/nltk/lib/python2.7/copy_reg.pyc in _reconstructor(cls, base, state)
     46 def _reconstructor(cls, base, state):
     47     if base is object:
---> 48         obj = object.__new__(cls)
     49     else:
     50         obj = base.__new__(cls, state)

TypeError: object.__new__(ConditionalProbDist) is not safe, use collections.defaultdict.__new__()

Start by reproducing the failure while loading taggers/hmm_treebank_pos_tagger/treebank.tagger.pickle.gz with nltk.data.find and pickle.load. Trace the tagger resource's loading and serialization compatibility, then verify that the resource loads without the reported TypeError.