diff --git a/db.go b/db.go
index bfc7f1f96..90d944f23 100644
--- a/db.go
+++ b/db.go
@@ -356,6 +356,7 @@ type chunkDesc struct {
 	firsTimestamp int64
 	lastTimestamp int64
 	lastValue     float64
+	numSamples    int
 
 	app chunks.Appender // Current appender for the chunks.
 }
@@ -374,6 +375,7 @@ func (cd *chunkDesc) append(ts int64, v float64) (err error) {
 
 	cd.lastTimestamp = ts
 	cd.lastValue = v
+	cd.numSamples++
 
 	return nil
 }
diff --git a/head.go b/head.go
index c09bf92e4..5f801e028 100644
--- a/head.go
+++ b/head.go
@@ -190,7 +190,9 @@ func (h *HeadBlock) appendBatch(samples []hashedSample) error {
 		newHashes []uint64
 	)
 
-	for _, s := range samples {
+	for i := range samples {
+		s := &samples[i]
+
 		cd, ref := h.get(s.hash, s.labels)
 		if cd != nil {
 			// TODO(fabxc): sample refs are only scoped within a block for
@@ -198,8 +200,8 @@ func (h *HeadBlock) appendBatch(samples []hashedSample) error {
 			s.ref = ref
 			continue
 		}
-
 		s.ref = uint32(len(h.descs) + len(newSeries))
+
 		newSeries = append(newSeries, s.labels)
 		newHashes = append(newHashes, s.hash)
 	}