diff --git a/tsdb/head.go b/tsdb/head.go
index 812f44d31f..0fde4c8452 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -1847,7 +1847,8 @@ func (s *memSeries) truncateChunksBefore(mint int64) (removed int) {
 }
 
 // append adds the sample (t, v) to the series. The caller also has to provide
-// the appendID for isolation.
+// the appendID for isolation. (The appendID can be zero, which results in no
+// isolation for this append.)
 func (s *memSeries) append(t int64, v float64, appendID uint64) (success, chunkCreated bool) {
 	// Based on Gorilla white papers this offers near-optimal compression ratio
 	// so anything bigger that this has diminishing returns and increases
@@ -1885,7 +1886,9 @@ func (s *memSeries) append(t int64, v float64, appendID uint64) (success, chunkC
 	s.sampleBuf[2] = s.sampleBuf[3]
 	s.sampleBuf[3] = sample{t: t, v: v}
 
-	s.txs.add(appendID)
+	if appendID > 0 {
+		s.txs.add(appendID)
+	}
 
 	return true, chunkCreated
 }
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index 0a274ca546..967f866651 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -1558,7 +1558,7 @@ func TestIsolationLowWatermarkMonotonous(t *testing.T) {
 	_, err = app1.Add(labels.FromStrings("foo", "bar"), 0, 0)
 	testutil.Ok(t, err)
 	testutil.Ok(t, app1.Commit())
-	testutil.Equals(t, uint64(1), hb.iso.lowWatermark())
+	testutil.Equals(t, uint64(1), hb.iso.lowWatermark(), "Low watermark should by 1 after 1st append.")
 
 	app1 = hb.Appender()
 	_, err = app1.Add(labels.FromStrings("foo", "bar"), 1, 1)
@@ -1580,3 +1580,17 @@ func TestIsolationLowWatermarkMonotonous(t *testing.T) {
 	is.Close()
 	testutil.Equals(t, uint64(3), hb.iso.lowWatermark(), "After read has finished (iso state closed), low watermark should jump to three.")
 }
+
+func TestIsolationAppendIDZeroIsNoop(t *testing.T) {
+	h, err := NewHead(nil, nil, nil, 1000, DefaultStripeSize)
+	testutil.Ok(t, err)
+	defer h.Close()
+
+	h.initTime(0)
+
+	s, _ := h.getOrCreate(1, labels.FromStrings("a", "1"))
+
+	ok, _ := s.append(0, 0, 0)
+	testutil.Assert(t, ok, "Series append failed.")
+	testutil.Equals(t, 0, s.txs.txIDCount, "Series should not have an appendID after append with appendID=0.")
+}
diff --git a/tsdb/isolation.go b/tsdb/isolation.go
index beaecfc48a..12a9bfe92c 100644
--- a/tsdb/isolation.go
+++ b/tsdb/isolation.go
@@ -110,7 +110,8 @@ func (i *isolation) State() *isolationState {
 	return isoState
 }
 
-// newAppendID increments the transaction counter and returns a new transaction ID.
+// newAppendID increments the transaction counter and returns a new transaction
+// ID. The first ID returned is 1.
 func (i *isolation) newAppendID() uint64 {
 	i.appendMtx.Lock()
 	defer i.appendMtx.Unlock()