diff --git a/grub-core/fs/iso9660.c b/grub-core/fs/iso9660.c
index 8c348b59a..8d480e602 100644
--- a/grub-core/fs/iso9660.c
+++ b/grub-core/fs/iso9660.c
@@ -551,6 +551,9 @@ grub_iso9660_mount (grub_disk_t disk)
   return data;
 
  fail:
+  if (grub_errno == GRUB_ERR_NONE)
+    grub_error (GRUB_ERR_BAD_FS, "not a ISO9660 filesystem");
+
   grub_free (data);
   return 0;
 }