diff --git a/src/t_zset.c b/src/t_zset.c
index ccf9962a1..99faae858 100644
--- a/src/t_zset.c
+++ b/src/t_zset.c
@@ -76,6 +76,7 @@ zskiplistNode *zslInsert(zskiplist *zsl, double score, robj *obj) {
     unsigned int rank[ZSKIPLIST_MAXLEVEL];
     int i, level;
 
+    redisAssert(!isnan(score));
     x = zsl->header;
     for (i = zsl->level-1; i >= 0; i--) {
         /* store rank that is crossed to reach the insert position */