diff --git a/Objects/setobject.c b/Objects/setobject.c
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -274,7 +274,6 @@
         entry->key = key;
         entry->hash = hash;
         so->used++;
-        Py_DECREF(dummy);
     } else {
         /* ACTIVE */
         Py_DECREF(key);
@@ -395,8 +394,6 @@
         } else if (entry->key == dummy_entry) {
             /* DUMMY */
             --i;
-            assert(entry->key == dummy);
-            Py_DECREF(entry->key);
         } else {
             /* ACTIVE */
             --i;
@@ -468,7 +465,6 @@
     if (entry->key == NULL  ||  entry->key == dummy)
         return DISCARD_NOTFOUND;
     old_key = entry->key;
-    Py_INCREF(dummy);
     entry->key = dummy;
     so->used--;
     Py_DECREF(old_key);
@@ -496,7 +492,6 @@
     if (entry->key == NULL  ||  entry->key == dummy)
         return DISCARD_NOTFOUND;
     old_key = entry->key;
-    Py_INCREF(dummy);
     entry->key = dummy;
     so->used--;
     Py_DECREF(old_key);
@@ -554,7 +549,8 @@
 #endif
         if (entry->key) {
             --fill;
-            Py_DECREF(entry->key);
+            if (entry->key != dummy)
+                Py_DECREF(entry->key);
         }
 #ifdef Py_DEBUG
         else
@@ -615,7 +611,8 @@
     for (entry = so->table; fill > 0; entry++) {
         if (entry->key) {
             --fill;
-            Py_DECREF(entry->key);
+            if (entry->key != dummy)
+                Py_DECREF(entry->key);
         }
     }
     if (so->table != so->smalltable)
@@ -788,7 +785,6 @@
         }
     }
     key = entry->key;
-    Py_INCREF(dummy);
     entry->key = dummy;
     so->used--;
     so->table[0].hash = i + 1;  /* next place to start */