diff --git a/head.go b/head.go
index 8bf0762bbc..4dfdd5b86b 100644
--- a/head.go
+++ b/head.go
@@ -521,15 +521,17 @@ func (a *headAppender) AddFast(ref string, t int64, v float64) error {
 	return nil
 }
 
-func (a *headAppender) createSeries() {
+func (a *headAppender) createSeries() error {
 	if len(a.newSeries) == 0 {
-		return
+		return nil
 	}
 	a.newLabels = make([]labels.Labels, 0, len(a.newSeries))
 	base0 := len(a.series)
 
 	a.mtx.RUnlock()
 	a.mtx.Lock()
+	defer a.mtx.RLock()
+	defer a.mtx.Unlock()
 
 	base1 := len(a.series)
 
@@ -549,8 +551,12 @@ func (a *headAppender) createSeries() {
 		a.create(l.hash, l.labels)
 	}
 
-	a.mtx.Unlock()
-	a.mtx.RLock()
+	// Write all new series to the WAL.
+	if err := a.wal.LogSeries(a.newLabels); err != nil {
+		return errors.Wrap(err, "WAL log series")
+	}
+
+	return nil
 }
 
 func (a *headAppender) Commit() error {
@@ -558,7 +564,9 @@ func (a *headAppender) Commit() error {
 	defer putHeadAppendBuffer(a.samples)
 	defer a.mtx.RUnlock()
 
-	a.createSeries()
+	if err := a.createSeries(); err != nil {
+		return err
+	}
 
 	// We have to update the refs of samples for series we just created.
 	for i := range a.samples {
@@ -568,11 +576,8 @@ func (a *headAppender) Commit() error {
 		}
 	}
 
-	// Write all new series and samples to the WAL and add it to the
+	// Write all new samples to the WAL and add them to the
 	// in-mem database on success.
-	if err := a.wal.LogSeries(a.newLabels); err != nil {
-		return errors.Wrap(err, "WAL log series")
-	}
 	if err := a.wal.LogSamples(a.samples); err != nil {
 		return errors.Wrap(err, "WAL log samples")
 	}