diff --git a/storage/interface.go b/storage/interface.go
index 5f0be9db9..3e8dd1086 100644
--- a/storage/interface.go
+++ b/storage/interface.go
@@ -422,7 +422,7 @@ type Labels interface {
 
 type SampleIterable interface {
 	// Iterator returns an iterator of the data of the series.
-	// The iterator passed as argument is for re-use.
+	// The iterator passed as argument is for re-use, if not nil.
 	// Depending on implementation, the iterator can
 	// be re-used or a new iterator can be allocated.
 	Iterator(chunkenc.Iterator) chunkenc.Iterator
diff --git a/storage/series.go b/storage/series.go
index 339beb2c9..377c060f7 100644
--- a/storage/series.go
+++ b/storage/series.go
@@ -287,7 +287,7 @@ func (s *seriesToChunkEncoder) Iterator(it chunks.Iterator) chunks.Iterator {
 	mint := int64(math.MaxInt64)
 	maxt := int64(math.MinInt64)
 
-	chks := []chunks.Meta{}
+	var chks []chunks.Meta
 	lcsi, existing := it.(*listChunkSeriesIterator)
 	if existing {
 		chks = lcsi.chks[:0]
diff --git a/tsdb/querier.go b/tsdb/querier.go
index 642e089aa..89e3d5719 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -426,7 +426,7 @@ func labelNamesWithMatchers(r IndexReader, matchers ...*labels.Matcher) ([]strin
 	return r.LabelNamesFor(postings...)
 }
 
-// These are the things fetched when we move from one series to another.
+// seriesData, used inside other iterators, are updated when we move from one series to another.
 type seriesData struct {
 	chks      []chunks.Meta
 	intervals tombstones.Intervals