diff --git a/libisofs/fs_image.c b/libisofs/fs_image.c
index 2c2eb65..39ec9e0 100644
--- a/libisofs/fs_image.c
+++ b/libisofs/fs_image.c
@@ -3964,7 +3964,7 @@ int iso_analyze_mbr(IsoImage *image, IsoDataSource *src, int flag)
         sub_type = 0;
         is_isohybrid = 1;
     } else if(ret == 2) {
-        sub_type = 0;
+        /* will become sub_type 0 if protective_label */
         is_grub2_mbr = 1;
     }
 
diff --git a/libisofs/libisofs.h b/libisofs/libisofs.h
index a16cddc..b3dbb3b 100644
--- a/libisofs/libisofs.h
+++ b/libisofs/libisofs.h
@@ -2220,8 +2220,9 @@ int iso_write_opts_set_fifo_size(IsoWriteOpts *opts, size_t fifo_size);
  *        bit10-13= System area sub type
  *              @since 1.2.4
  *              With type 0:
- *                bit0 ... MBR with partition start at block 1
- *                bit1 ... ISOLINUX isohybrid MBR
+ *                if options bit0 ... MBR with partition start at block 1
+ *                if options bit1 ... ISOLINUX isohybrid MBR
+ *                else:
  *                0 = no particular sub type, use unaltered
  *                1 = CHRP: A single MBR partition of type 0x96 covers the
  *                          ISO image. Not compatible with any other feature