Error in Python NLTK

[subhabangalore at gmail.com]

Dear Group,

I was using a package named NLTK in Python. 

I was trying to write a code given in section 3.8 of 

http://docs.huihoo.com/nltk/0.9.5/guides/tag.html.

Here, in the >>> test = ['up', 'down', 'up'] if I put more than 3 values and trying to write the reciprocal codes, like,

            sequence = [(t, None) for t in test] and print '%.3f' % (model.probability(sequence))

I am getting an error as, 

Traceback (most recent call last): File "", line 1, in model.probability(sequence) File "C:\Python27\lib\site-packages\nltk\tag\hmm.py", line 228, in probability return 2**(self.log_probability(self._transform.transform(sequence))) File "C:\Python27\lib\site-packages\nltk\tag\hmm.py", line 259, in log_probability alpha = self._forward_probability(sequence) File "C:\Python27\lib\site-packages\nltk\tag\hmm.py", line 694, in _forward_probability alpha[0, i] = self._priors.logprob(state) + \ File "C:\Python27\lib\site-packages\nltk\probability.py", line 689, in logprob elif self._prob_dict[sample] == 0: return _NINF ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

If any learned member may kindly assist me how may I solve the issue. 

Regards, 
Subhabrata.