diff --git a/builtin-diff.c b/builtin-diff.c
index 1c535b1..a659020 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -137,7 +137,7 @@
 	int cached = 0;
 	while (1 < argc) {
 		const char *arg = argv[1];
-		if (!strcmp(arg, "--index") || !strcmp(arg, "--cached"))
+		if (!strcmp(arg, "--cached"))
 			cached = 1;
 		else
 			usage(builtin_diff_usage);