TypeError: expected string or buffer

Traceback (most recent call last):
  File "gensim_test.py", line 34, in <module>
    first_poem_letters_only = re.sub("[^a-zA-Z]", '', first_poem)
  File "/usr/lib/python2.7/re.py", line 155, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or buffer