diff --git a/luaa.c b/luaa.c
index e25dea50c..9ce3a8a03 100644
--- a/luaa.c
+++ b/luaa.c
@@ -192,7 +192,7 @@ static int
 luaA_mbstrlen(lua_State *L)
 {
     const char *cmd = luaL_checkstring(L, 1);
-    lua_pushnumber(L, mbstowcs(NULL, NONULL(cmd), 0));
+    lua_pushnumber(L, (ssize_t) mbstowcs(NULL, NONULL(cmd), 0));
     return 1;
 }