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) {
@@ -4329,37 +4330,53 @@
     }
     assert(PyList_Check(list));
     newobj = PyWeakref_NewRef((PyObject *)type, NULL);
-    i = PyList_GET_SIZE(list);
+    result = PyList_Append(list, newobj);
+    Py_DECREF(newobj);
+    return result;
+}
+
+static void
+remove_subclass(PyTypeObject *base, PyTypeObject *type)
+{
+    Py_ssize_t i, k, n;
+    PyObject *list, *ref, *target;
+
+    list = base->tp_subclasses;
+    if (list == NULL) {
+        return;
+    }
+    assert(PyList_Check(list));
+    i = k = n = 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;
+        target = PyWeakref_GET_OBJECT(ref);
+        if (target == Py_None || target == (PyObject*) type) {
+            /* swap the item with the last one */
+            k--;
+            if (i != k) {
+                PyObject *tmp = PyList_GET_ITEM(list, k);
+                PyList_SET_ITEM(list, k, ref) ;
+                PyList_SET_ITEM(list, i, tmp);
+            }
+            if (target == (PyObject *) type)
+                break;
+        }
+    }
+    if (k < n)
+        PyList_SetSlice(list, k, n, NULL);
 }
 
 static void
-remove_subclass(PyTypeObject *base, PyTypeObject *type)
-{
+remove_all_subclasses(PyTypeObject *type)
+{
+    PyObject *bases = type->tp_bases;
     Py_ssize_t i;
-    PyObject *list, *ref;
-
-    list = base->tp_subclasses;
-    if (list == NULL) {
-        return;
-    }
-    assert(PyList_Check(list));
-    i = PyList_GET_SIZE(list);
-    while (--i >= 0) {
-        ref = PyList_GET_ITEM(list, i);
-        assert(PyWeakref_CheckRef(ref));
-        if (PyWeakref_GET_OBJECT(ref) == (PyObject*)type) {
-            /* this can't fail, right? */
-            PySequence_DelItem(list, i);
-            return;
+    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);
         }
     }
 }