filterstring() and filterunicode() in Python/bltinmodule.c blindly assumed that tp_as_sequence->sq_item always returns a str or unicode object. This might fail with str or unicode subclasses. This patch checks whether the object returned from __getitem__ is a str/unicode object and raises a TypeError if not (and the filter function returned true). Furthermore the result for __getitem__ can be more than one character long, so checks for enough memory have to be done.