diff -r 514571268743 Objects/dictobject.c
--- a/Objects/dictobject.c	Tue Feb 07 11:27:09 2017 +0300
+++ b/Objects/dictobject.c	Sat Feb 11 03:15:04 2017 +0900
@@ -3890,26 +3890,24 @@
             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;
+            int was_shared = (cached == ((PyDictObject *)dict)->ma_keys);
             res = PyDict_SetItem(dict, key, value);
             /* 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.
              */
-            if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {
+            if (was_shared &&
+                    (cached = CACHED_KEYS(tp)) != NULL &&
+                    cached != ((PyDictObject *)dict)->ma_keys) {
                 if (cached->dk_refcnt == 1) {
                     CACHED_KEYS(tp) = make_keys_shared(dict);
                 } else {
                     CACHED_KEYS(tp) = NULL;
                 }
                 DK_DECREF(cached);