diff --git a/include/sys/zfs_ioctl.h b/include/sys/zfs_ioctl.h
index 06e3cb24a4..317622f7c7 100644
--- a/include/sys/zfs_ioctl.h
+++ b/include/sys/zfs_ioctl.h
@@ -105,6 +105,7 @@ typedef enum drr_headertype {
 #define	DMU_BACKUP_FEATURE_COMPRESSED		(1 << 22)
 #define	DMU_BACKUP_FEATURE_LARGE_DNODE		(1 << 23)
 #define	DMU_BACKUP_FEATURE_RAW			(1 << 24)
+/* flag #25 is reserved for the ZSTD compression feature */
 
 /*
  * Mask of all supported backup features