diff --git a/src/os/src/detail/osString.c b/src/os/src/detail/osString.c
index 2c49797d83dd4bb53b2fef69091e28be7194461e..fdc70b667e08face21554c99e2109bc49cf41cbf 100644
--- a/src/os/src/detail/osString.c
+++ b/src/os/src/detail/osString.c
@@ -59,6 +59,9 @@ bool taosMbsToUcs4(char *mbs, size_t mbsLength, char *ucs4, int32_t ucs4_max_len
   iconv_close(cd);
   if (len != NULL) {
     *len = (int32_t)(ucs4_max_len - outLeft);
+    if (*len < 0) {
+      return false;
+    }
   }
 
   return true;