diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index 3cd00729ab..8c0f698eae 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -438,7 +438,7 @@ func (w *Writer) AddSeries(ref storage.SeriesRef, lset labels.Labels, chunks ...
 		return err
 	}
 	if labels.Compare(lset, w.lastSeries) <= 0 {
-		return fmt.Errorf("out-of-order series added with label set %q", lset)
+		return fmt.Errorf("out-of-order series added with label set %q, last label set %q", lset, w.lastSeries)
 	}
 
 	if ref < w.lastSeriesRef && !w.lastSeries.IsEmpty() {