diff -r 0f3ebeb389fe Objects/dictobject.c
--- a/Objects/dictobject.c	Thu Feb 02 19:26:48 2017 +0000
+++ b/Objects/dictobject.c	Sat Feb 04 19:10:49 2017 +0900
@@ -4373,21 +4373,18 @@
             if (dict == NULL)
                 return -1;
             *dictptr = dict;
         }
         if (value == NULL) {
             res = PyDict_DelItem(dict, key);
-            if (cached != ((PyDictObject *)dict)->ma_keys) {
-                CACHED_KEYS(tp) = NULL;
-                DK_DECREF(cached);
-            }
         }
         else {
             int was_shared = cached == ((PyDictObject *)dict)->ma_keys;
             res = PyDict_SetItem(dict, key, value);
-            if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {
+            if (was_shared && cached != ((PyDictObject *)dict)->ma_keys
+                    && cached == CACHED_KEYS(tp)) {
                 /* PyDict_SetItem() may call dictresize and convert split table
                  * into combined table.  In such case, convert it to split
                  * table again and update type's shared key only when this is
                  * the only dict sharing key with the type.
                  *
                  * This is to allow using shared key in class like this:
@@ -4400,16 +4397,19 @@
                  *     a = C()
                  */
                 if (cached->dk_refcnt == 1) {
                     CACHED_KEYS(tp) = make_keys_shared(dict);
                 }
                 else {
-                    CACHED_KEYS(tp) = NULL;
+                    // CACHED_KEYS(tp) = NULL allows further attemt to
+                    // use new shared key.  Use Py_EMPTY_KEYS to prevent it.
+                    DK_INCREF(Py_EMPTY_KEYS);
+                    CACHED_KEYS(tp) = Py_EMPTY_KEYS;
                 }
                 DK_DECREF(cached);
-                if (CACHED_KEYS(tp) == NULL && PyErr_Occurred())
+                if (CACHED_KEYS(tp) == NULL && PyErr_Occurred()) 
                     return -1;
             }
         }
     } else {
         dict = *dictptr;
         if (dict == NULL) {