diff --git a/fast-import.c b/fast-import.c
index 6707a66471..b2030ccfb4 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -3342,18 +3342,21 @@ static void parse_argv(void)
 		if (*a != '-' || !strcmp(a, "--"))
 			break;
 
-		if (parse_one_option(a + 2))
+		if (!skip_prefix(a, "--", &a))
+			die("unknown option %s", a);
+
+		if (parse_one_option(a))
 			continue;
 
-		if (parse_one_feature(a + 2, 0))
+		if (parse_one_feature(a, 0))
 			continue;
 
-		if (starts_with(a + 2, "cat-blob-fd=")) {
-			option_cat_blob_fd(a + 2 + strlen("cat-blob-fd="));
+		if (skip_prefix(a, "cat-blob-fd=", &a)) {
+			option_cat_blob_fd(a);
 			continue;
 		}
 
-		die("unknown option %s", a);
+		die("unknown option --%s", a);
 	}
 	if (i != global_argc)
 		usage(fast_import_usage);