Don't be so hasty. If PyInt_AsLong() raises an error, don't set ValueError.

Objects/typeobject.c

@@ -2972,6 +2972,8 @@ slot_sq_length(PyObject *self)
 		return -1;
 	len = (int)PyInt_AsLong(res);
 	Py_DECREF(res);
+	if (len == -1 && PyErr_Occurred())
+		return -1;
 	if (len < 0) {
 		PyErr_SetString(PyExc_ValueError, 
 				"__len__() should return >= 0");