diff --git a/diff.c b/diff.c
index 9a5c77c..0e2e872 100644
--- a/diff.c
+++ b/diff.c
@@ -3310,7 +3310,7 @@
 	else if ((argcount = parse_long_opt("output", av, &optarg))) {
 		options->file = fopen(optarg, "w");
 		if (!options->file)
-			die_errno("Could not open '%s'", arg + strlen("--output="));
+			die_errno("Could not open '%s'", optarg);
 		options->close_file = 1;
 		return argcount;
 	} else