From the discussion of issue 643841:

>>> class Demo:
...   def __index__(self):
...     return 1
...
>>> a = Demo()
>>> b = weakref.proxy(a)
>>> operator.index(a)
1
>>> operator.index(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'weakproxy' object cannot be interpreted as an index

The weakref proxy types need to be updated to delegate 2.5's new
tp_index slot.

Marking as easy - it should be possible to just look at what the code in
Objects\weakref.c is already doing for slots like tp_len and tp_int and
do the same thing for tp_index.

Fixed in r63500, together with the floor/truediv slots.