an error caused by brackets

[LTEnjoy]

Hi, I ran into an error when ran the code:
words = ['(', 'There', 'is', 'no', 'doubt', ')'] and I got the error as follow:
IndexError: list index out of range
What is mystrious is that when I deleted the brackets '(' and ')', it worked.
Could you tell me how can I rewrite the code to make it work even with brackets?

[yzhangcs]

@LTEnjoy It's a bug of NLTK. A way is to replace the brackets with other symbols like '['/']'.