diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -1170,9 +1170,26 @@
             self.assertEqual(ns, ns_roundtrip, pname)
 
 
+class SharedKeyTests(unittest.TestCase):
+
+    def test_subclasses(self):
+        # Verify that subclasses can share keys (per PEP 412)
+        class A:
+            pass
+        class B(A):
+            pass
+
+        a, b = A(), B()
+        self.assertEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(b)))
+        a.x, a.y, a.z, a.w = range(4)
+        self.assertNotEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(b)))
+        a2 = A()
+        self.assertEqual(sys.getsizeof(vars(a)), sys.getsizeof(vars(a2)))
+
+
 def test_main():
     run_unittest(TypesTests, MappingProxyTests, ClassCreationTests,
-                 SimpleNamespaceTests)
+                 SimpleNamespaceTests, SharedKeyTests)
 
 if __name__ == '__main__':
     test_main()
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2440,6 +2440,9 @@
             type->tp_dictoffset = slotoffset;
         slotoffset += sizeof(PyObject *);
     }
+    else if (!type->tp_dictoffset) {
+        type->tp_dictoffset = base->tp_dictoffset;
+    }
     if (type->tp_dictoffset) {
         et->ht_cached_keys = _PyDict_NewKeysForClass();
     }
@@ -4342,7 +4345,6 @@
 
     COPYVAL(tp_itemsize);
     COPYVAL(tp_weaklistoffset);
-    COPYVAL(tp_dictoffset);
 
     /* Setup fast subclass flags */
     if (PyType_IsSubtype(base, (PyTypeObject*)PyExc_BaseException))