$ python invalid_tags.py
Traceback (most recent call last):
  File "invalid_tags.py", line 17, in <module>
    print strip_tags(html, invalid_tags)
  File "invalid_tags.py", line 10, in strip_tags
    c = strip_tags(c, invalid_tags)
  File "invalid_tags.py", line 4, in strip_tags
    soup = BeautifulSoup(html)
  File "/usr/lib/python2.7/dist-packages/BeautifulSoup.py", line 1519, in __init__
    BeautifulStoneSoup.__init__(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/BeautifulSoup.py", line 1140, in __init__
    markup = markup.read()
TypeError: 'NoneType' object is not callable