diff --git a/usr.sbin/makefs/cd9660.c b/usr.sbin/makefs/cd9660.c
index b5be46906b09..e05e52ac32c9 100644
--- a/usr.sbin/makefs/cd9660.c
+++ b/usr.sbin/makefs/cd9660.c
@@ -296,8 +296,8 @@ cd9660_parse_opts(const char *option, fsinfo_t *fsopts)
 	int	rv;
 	/* Set up allowed options - integer options ONLY */
 	option_t cd9660_options[] = {
-		{ "l", &diskStructure.isoLevel, 1, 3, "ISO Level" },
-		{ "isolevel", &diskStructure.isoLevel, 1, 3, "ISO Level" },
+		{ "l", &diskStructure.isoLevel, 1, 2, "ISO Level" },
+		{ "isolevel", &diskStructure.isoLevel, 1, 2, "ISO Level" },
 		{ "verbose",  &diskStructure.verbose_level, 0, 2,
 		  "Turns on verbose output" },
 		{ "v", &diskStructure.verbose_level, 0 , 2,
@@ -1055,6 +1055,7 @@ cd9660_rename_filename(cd9660node *iter, int num, int delete_chars)
 	if (diskStructure.verbose_level > 0)
 		printf("Rename_filename called\n");
 
+	assert(1 <= diskStructure.isoLevel && diskStructure.isoLevel <= 2);
 	/* TODO : A LOT of chanes regarding 8.3 filenames */
 	if (diskStructure.isoLevel == 1)
 		maxlength = 8;
@@ -1730,6 +1731,7 @@ cd9660_joliet_convert_filename(const char *oldname, char *newname, int is_file)
 static int
 cd9660_convert_filename(const char *oldname, char *newname, int is_file)
 {
+	assert(1 <= diskStructure.isoLevel && diskStructure.isoLevel <= 2);
 	/* NEW */
 	cd9660_filename_conversion_functor conversion_function = 0;
 	if (diskStructure.isoLevel == 1)