$ 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