diff --git a/head.go b/head.go
index 1cc654bd3..0fbf747a3 100644
--- a/head.go
+++ b/head.go
@@ -298,23 +298,25 @@ func (h *headBlock) appendBatch(samples []hashedSample) (int, error) {
 	// be no further failures.
 	if len(newSeries) > 0 {
 		newLabels = make([]labels.Labels, 0, len(newSeries))
+		base0 := len(h.series)
 
 		h.mtx.RUnlock()
 		h.mtx.Lock()
 
-		base := len(h.series)
+		base1 := len(h.series)
 		i := 0
 
 		for hash, ser := range newSeries {
 			lset := ser[0].labels
 			// We switched locks and have to re-validate that the series were not
 			// created by another goroutine in the meantime.
-			ms := h.get(hash, lset)
-			if ms != nil {
-				for _, s := range ser {
-					s.ref = ms.ref
+			if base1 != base0 {
+				if ms := h.get(hash, lset); ms != nil {
+					for _, s := range ser {
+						s.ref = ms.ref
+					}
+					continue
 				}
-				continue
 			}
 			// Series is still new.
 			newLabels = append(newLabels, lset)
@@ -322,7 +324,7 @@ func (h *headBlock) appendBatch(samples []hashedSample) (int, error) {
 			h.create(hash, lset)
 			// Set sample references to the series we just created.
 			for _, s := range ser {
-				s.ref = uint32(base + i)
+				s.ref = uint32(base1 + i)
 			}
 			i++
 		}
@@ -343,9 +345,11 @@ func (h *headBlock) appendBatch(samples []hashedSample) (int, error) {
 	)
 	for _, s := range samples {
 		ser := h.series[s.ref]
+
 		ser.mtx.Lock()
 		ok := ser.append(s.t, s.v)
 		ser.mtx.Unlock()
+
 		if !ok {
 			total--
 			continue