diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index 8172b81ce..09fb737bf 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -1557,6 +1557,7 @@ func (r *Reader) LabelNamesFor(ctx context.Context, postings Postings) ([]string
 	i := 0
 	for postings.Next() {
 		id := postings.At()
+		i++
 
 		if i%checkContextEveryNIterations == 0 && ctx.Err() != nil {
 			return nil, ctx.Err()