diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 7433ae929fdc..2e0832d70406 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6595,11 +6595,13 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 		if (patch_the_first_stripe_for_dev_replace) {
 			smap->dev = dev_replace->tgtdev;
 			smap->physical = physical_to_patch_in_first_stripe;
-			*mirror_num_ret = map->num_stripes + 1;
+			if (mirror_num_ret)
+				*mirror_num_ret = map->num_stripes + 1;
 		} else {
 			set_io_stripe(smap, map, stripe_index, stripe_offset,
 				      stripe_nr);
-			*mirror_num_ret = mirror_num;
+			if (mirror_num_ret)
+				*mirror_num_ret = mirror_num;
 		}
 		*bioc_ret = NULL;
 		ret = 0;