diff --git a/src/os/src/detail/osString.c b/src/os/src/detail/osString.c
index 04c81cda3f474cd9adcf1c6e60526a06cf411d06..59f47b4815badd8411b54ea997bf6b46f3a18a27 100644
--- a/src/os/src/detail/osString.c
+++ b/src/os/src/detail/osString.c
@@ -58,7 +58,7 @@ bool taosMbsToUcs4(char *mbs, size_t mbsLength, char *ucs4, int32_t ucs4_max_len
 
   iconv_close(cd);
   if (len != NULL) {
-    *len = ucs4_max_len - outLeft;
+    *len = (int32_t)(ucs4_max_len - outLeft);
   }
 
   return true;