iteration of ConcatenatedCorpusView bug in NLTK

gistfile1.txt

import nltk

wsj = nltk.corpus.treebank.tagged_words(simplify_tags=True)
cdf = nltk.ConditionalFreqDist((tag, word) for (word, tag) in wsj)

wordlist = cdf['VN'].keys()

# Bug 1: strange iteration exceptions in the for loop
# Solution: wsj is a custom NLTK data type "ConcatenatedCorpusView"
# convert wsj into a native python type "list" for better iteration.
# I am guessing ConcatenatedCorpusView chokes on empty tuples (in this case
# at index 17).
wsj_list = list(wsj)

# Bug 2: repeated words return the index of the first word only
# Solution: Use enumerate() to index

for ndx, (word, tag) in enumerate(wsj_list):
    if word in wordlist and tag == 'VN':
        print wsj_list[ndx-1:ndx+1]