diff --git a/Objects/typeobject.c b/Objects/typeobject.c
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -438,6 +438,7 @@
 static int compatible_for_assignment(PyTypeObject *, PyTypeObject *, char *);
 static int add_subclass(PyTypeObject*, PyTypeObject*);
 static void remove_subclass(PyTypeObject *, PyTypeObject *);
+static void remove_all_subclasses(PyTypeObject *type);
 static void update_all_slots(PyTypeObject *);
 
 typedef int (*update_callback)(PyTypeObject *, void *);
@@ -2713,6 +2714,7 @@
     et = (PyHeapTypeObject *)type;
     Py_XDECREF(type->tp_base);
     Py_XDECREF(type->tp_dict);
+    remove_all_subclasses(type);
     Py_XDECREF(type->tp_bases);
     Py_XDECREF(type->tp_mro);
     Py_XDECREF(type->tp_cache);
@@ -4317,9 +4319,8 @@
 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) {
@@ -4329,13 +4330,6 @@
     }
     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;
@@ -4345,7 +4339,7 @@
 remove_subclass(PyTypeObject *base, PyTypeObject *type)
 {
     Py_ssize_t i;
-    PyObject *list, *ref;
+    PyObject *list, *ref, *target;
 
     list = base->tp_subclasses;
     if (list == NULL) {
@@ -4356,14 +4350,36 @@
     while (--i >= 0) {
         ref = PyList_GET_ITEM(list, i);
         assert(PyWeakref_CheckRef(ref));
-        if (PyWeakref_GET_OBJECT(ref) == (PyObject*)type) {
+        target = PyWeakref_GET_OBJECT(ref);
+        if (target == Py_None || target == (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;
         }
     }
 }
 
+static void
+remove_all_subclasses(PyTypeObject *type)
+{
+    PyObject *bases = type->tp_bases;
+    int i;
+    if (bases) {
+        for(i = 0; i < PyTuple_GET_SIZE(bases); i++) {
+            PyObject *base = PyTuple_GET_ITEM(bases, i);
+            if (PyType_Check(base))
+                remove_subclass((PyTypeObject*)base, type);
+        }
+    }
+}
+
 static int
 check_num_args(PyObject *ob, int n)
 {