diff --git a/degesch.c b/degesch.c
index 08ffb5e..30fa19b 100644
--- a/degesch.c
+++ b/degesch.c
@@ -8401,7 +8401,7 @@ lua_plugin_check_utf8 (lua_State *L, int arg)
 {
 	size_t len;
 	const char *s = luaL_checklstring (L, arg, &len);
-	luaL_argcheck (L, utf8_validate (s, len), 1, "must be valid UTF-8");
+	luaL_argcheck (L, utf8_validate (s, len), arg, "must be valid UTF-8");
 	return s;
 }