diff --git a/cmd/arg.c b/cmd/arg.c
index f7e4963..9e625ac 100644
--- a/cmd/arg.c
+++ b/cmd/arg.c
@@ -127,7 +127,7 @@ void z__b_get_arg_usage_string(
 
 	if (optional) {
 		b_string_append_cstrf(
-			out, colour ? F_GREEN "[%s]" : "[%s]", arg->arg_name);
+			out, colour ? F_GREEN "[[%s]" : "[[%s]", arg->arg_name);
 	} else {
 		b_string_append_cstrf(
 			out, colour ? F_GREEN "<%s>" : "<%s>", arg->arg_name);