diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
index c54091a28909..ede0b480e7d4 100644
--- a/fs/bcachefs/snapshot.c
+++ b/fs/bcachefs/snapshot.c
@@ -146,8 +146,9 @@ bool __bch2_snapshot_is_ancestor(struct bch_fs *c, u32 id, u32 ancestor)
 		goto out;
 	}
 
-	while (id && id < ancestor - IS_ANCESTOR_BITMAP)
-		id = get_ancestor_below(t, id, ancestor);
+	if (likely(ancestor >= IS_ANCESTOR_BITMAP))
+		while (id && id < ancestor - IS_ANCESTOR_BITMAP)
+			id = get_ancestor_below(t, id, ancestor);
 
 	ret = id && id < ancestor
 		? test_ancestor_bitmap(t, id, ancestor)