diff --git a/src/t_set.c b/src/t_set.c
index 0ba8335aa..6e8e1f4e0 100644
--- a/src/t_set.c
+++ b/src/t_set.c
@@ -820,6 +820,7 @@ void sunionDiffGenericCommand(redisClient *c, robj **setkeys, int setnum, robj *
         while((ele = setTypeNextObject(si)) != NULL) {
             for (j = 1; j < setnum; j++) {
                 if (!sets[j]) continue; /* no key is an empty set. */
+                if (sets[j] == sets[0]) break; /* same set! */
                 if (setTypeIsMember(sets[j],ele)) break;
             }
             if (j == setnum) {