diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5ec000813f04..436e15e3759d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6399,7 +6399,8 @@ int __btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
 	    (!need_full_stripe(op) || !dev_replace_is_ongoing ||
 	     !dev_replace->tgtdev)) {
 		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;
 		goto out;