diff --git a/plugins/check_http.c b/plugins/check_http.c
index 038ac68a..2f693328 100644
--- a/plugins/check_http.c
+++ b/plugins/check_http.c
@@ -348,7 +348,7 @@ process_arguments (int argc, char **argv)
 
 		switch (c) {
 		case '?': /* usage */
-			usage2 ("unknown argument", optarg);
+			usage3 ("unknown argument", optopt);
 			break;
 		case 'h': /* help */
 			print_help ();