diff -r 975e03b0aea6 Objects/dictobject.c
--- a/Objects/dictobject.c	Tue Feb 07 11:27:48 2017 +0300
+++ b/Objects/dictobject.c	Wed Feb 08 15:20:46 2017 +0900
@@ -4373,21 +4373,24 @@
             if (dict == NULL)
                 return -1;
             *dictptr = dict;
         }
         if (value == NULL) {
             res = PyDict_DelItem(dict, key);
-            if (cached != ((PyDictObject *)dict)->ma_keys) {
+            // key sharing dict doesn't allow deletion.
+            if ((cached = CACHED_KEYS(tp)) != NULL) {
                 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);
-            if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {
+            if (res == 0 && was_shared &&
+                    (cached = CACHED_KEYS(tp)) != NULL &&
+                    cached != ((PyDictObject *)dict)->ma_keys) {
                 /* 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: