```
>>> class zero(object):
...     def __index__(self):
...         return 0
... 
>>> z = zero()
>>> import re
>>> p = re.compile('(a)b')
>>> m = p.match('ab')
>>> m.group(0)
'ab'
>>> m.group(z)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group
```