Starting at line 1223 in classobject.c, you can find this code:

    if (item == NULL)
        arg = PyInt_FromSsize_t(i);
    else
        arg = Py_BuildValue("(nO)", i, item);
    if (arg == NULL) {
        Py_DECREF(func);
        return -1;
    }
    res = PyEval_CallObject(func, arg);

If item is NULL, arg will be assigned an int object. Otherwise, it will receive a tuple. Only the second case works in the subsequent call to PyEval_CallObject(), i.e. arg must always be assigned an argument tuple.

A quick fix would be to call Py_BuildValue("(n)", i) in the first case. The code just did a getattr(), so this is not performance critical anymore.

I found this bug because the test_class.py test suite was failing in Cython.