According to http://docs.python.org/api/object.html,

"""
PyObject* PyObject_Call(	PyObject *callable_object, PyObject *args,
PyObject *kw)
    Return value: New reference.
    Call a callable Python object callable_object, with arguments given
by the tuple args, and named arguments given by the dictionary kw. If no
named arguments are needed, kw may be NULL. args must not be NULL, use
an empty tuple if no arguments are needed. Returns the result of the
call on success, or NULL on failure. 
"""

passing Py_None as kw is not allowed.  Interestingly, the documentation
also says that "args must not be NULL," while the code is clearly more
forgiving.