diff --git a/Objects/typeobject.c b/Objects/typeobject.c
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -4302,9 +4302,8 @@
 static int
 add_subclass(PyTypeObject *base, PyTypeObject *type)
 {
-    Py_ssize_t i;
     int result;
-    PyObject *list, *ref, *newobj;
+    PyObject *list, *newobj;
 
     list = base->tp_subclasses;
     if (list == NULL) {
@@ -4314,13 +4313,6 @@
     }
     assert(PyList_Check(list));
     newobj = PyWeakref_NewRef((PyObject *)type, NULL);
-    i = PyList_GET_SIZE(list);
-    while (--i >= 0) {
-        ref = PyList_GET_ITEM(list, i);
-        assert(PyWeakref_CheckRef(ref));
-        if (PyWeakref_GET_OBJECT(ref) == Py_None)
-            return PyList_SetItem(list, i, newobj);
-    }
     result = PyList_Append(list, newobj);
     Py_DECREF(newobj);
     return result;
@@ -4342,8 +4334,15 @@
         ref = PyList_GET_ITEM(list, i);
         assert(PyWeakref_CheckRef(ref));
         if (PyWeakref_GET_OBJECT(ref) == (PyObject*)type) {
+            /* swap the item with the last one */
+            Py_ssize_t j = PyList_GET_SIZE(list)-1;
+            if (i != j) {
+                PyObject *tmp = PyList_GET_ITEM(list, j);
+                PyList_SET_ITEM(list, j, ref) ;
+                PyList_SET_ITEM(list, i, tmp);
+            }
             /* this can't fail, right? */
-            PySequence_DelItem(list, i);
+            PySequence_DelItem(list, j);
             return;
         }
     }