diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 9662d95..96fc7cd 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2972,6 +2972,11 @@
 		return -1;
 	len = (int)PyInt_AsLong(res);
 	Py_DECREF(res);
+	if (len < 0) {
+		PyErr_SetString(PyExc_ValueError, 
+				"__len__() should return >= 0");
+		return -1;
+	}
 	return len;
 }