diff --git a/migration/block.c b/migration/block.c
index 1941bc2402bdb17e5136cafe82003c520a9cfa2f..67412282003500c45c5e74d38ee23e6041035203 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -276,6 +276,8 @@ static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
     if (bmds->shared_base) {
         qemu_mutex_lock_iothread();
         aio_context_acquire(blk_get_aio_context(bb));
+        /* Skip unallocated sectors; intentionally treats failure as
+         * an allocated sector */
         while (cur_sector < total_sectors &&
                !bdrv_is_allocated(blk_bs(bb), cur_sector,
                                   MAX_IS_ALLOCATED_SEARCH, &nr_sectors)) {