diff --git a/src/sds.c b/src/sds.c
index 036d714d4..e722abbca 100644
--- a/src/sds.c
+++ b/src/sds.c
@@ -682,6 +682,7 @@ sds sdscatfmt(sds s, char const *fmt, ...) {
         switch(*f) {
         case '%':
             next = *(f+1);
+            if (next == '\0') break;
             f++;
             switch(next) {
             case 's':