diff --git a/tsdb/engine/tsm1/cache_test.go b/tsdb/engine/tsm1/cache_test.go
index d5283024d6..21512704bf 100644
--- a/tsdb/engine/tsm1/cache_test.go
+++ b/tsdb/engine/tsm1/cache_test.go
@@ -354,7 +354,7 @@ func mustMarshalEntry(entry WALEntry) (WalEntryType, []byte) {
 
 	b, err := entry.Encode(bytes)
 	if err != nil {
-		panic("error encoding")
+		panic(fmt.Sprintf("error encoding: %v", err))
 	}
 
 	return entry.Type(), snappy.Encode(b, b)
diff --git a/tsdb/engine/tsm1/wal.go b/tsdb/engine/tsm1/wal.go
index 29c4a7e0a6..6d5d91ce1c 100644
--- a/tsdb/engine/tsm1/wal.go
+++ b/tsdb/engine/tsm1/wal.go
@@ -366,6 +366,13 @@ func (w *WriteWALEntry) Encode(dst []byte) ([]byte, error) {
 
 	for k, v := range w.Values {
 
+		// Make sure we have enough space in our buf before copying.  If not,
+		// grow the buf.
+		if len(dst[:n])+2+len(k)+len(v)*8+4 > len(dst) {
+			grow := make([]byte, len(dst)*2)
+			dst = append(dst, grow...)
+		}
+
 		switch v[0].Value().(type) {
 		case float64:
 			dst[n] = float64EntryType
@@ -380,19 +387,19 @@ func (w *WriteWALEntry) Encode(dst []byte) ([]byte, error) {
 		}
 		n++
 
-		// Make sure we have enough space in our buf before copying.  If not,
-		// grow the buf.
-		if len(k)+2+len(v)*8+4 > len(dst)-n {
-			grow := make([]byte, len(dst)*2)
-			dst = append(dst, grow...)
-		}
-
 		n += copy(dst[n:], u16tob(uint16(len(k))))
 		n += copy(dst[n:], []byte(k))
 
 		n += copy(dst[n:], u32tob(uint32(len(v))))
 
 		for _, vv := range v {
+
+			// Grow our slice if needed
+			if len(dst[:n])+16 > len(dst) {
+				grow := make([]byte, len(dst)*2)
+				dst = append(dst, grow...)
+			}
+
 			n += copy(dst[n:], u64tob(uint64(vv.Time().UnixNano())))
 			switch t := vv.Value().(type) {
 			case float64:
diff --git a/tsdb/engine/tsm1/wal_test.go b/tsdb/engine/tsm1/wal_test.go
index bea473a654..ad6d70b9f4 100644
--- a/tsdb/engine/tsm1/wal_test.go
+++ b/tsdb/engine/tsm1/wal_test.go
@@ -70,6 +70,62 @@ func TestWALWriter_WritePoints_Single(t *testing.T) {
 	}
 }
 
+func TestWALWriter_WritePoints_LargeBatch(t *testing.T) {
+	dir := MustTempDir()
+	defer os.RemoveAll(dir)
+	f := MustTempFile(dir)
+	w := tsm1.NewWALSegmentWriter(f)
+
+	var points []tsm1.Value
+	for i := 0; i < 100000; i++ {
+		points = append(points, tsm1.NewValue(time.Unix(int64(i), 0), int64(1)))
+	}
+
+	values := map[string][]tsm1.Value{
+		"cpu,host=A,server=01,foo=bar,tag=really-long#!~#float": points,
+		"mem,host=A,server=01,foo=bar,tag=really-long#!~#float": points,
+	}
+
+	entry := &tsm1.WriteWALEntry{
+		Values: values,
+	}
+
+	if err := w.Write(mustMarshalEntry(entry)); err != nil {
+		fatal(t, "write points", err)
+	}
+
+	if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+		fatal(t, "seek", err)
+	}
+
+	r := tsm1.NewWALSegmentReader(f)
+
+	if !r.Next() {
+		t.Fatalf("expected next, got false")
+	}
+
+	we, err := r.Read()
+	if err != nil {
+		fatal(t, "read entry", err)
+	}
+
+	e, ok := we.(*tsm1.WriteWALEntry)
+	if !ok {
+		t.Fatalf("expected WriteWALEntry: got %#v", e)
+	}
+
+	for k, v := range e.Values {
+		for i, vv := range v {
+			if got, exp := vv.String(), values[k][i].String(); got != exp {
+				t.Fatalf("points mismatch: got %v, exp %v", got, exp)
+			}
+		}
+	}
+
+	if n := r.Count(); n != MustReadFileSize(f) {
+		t.Fatalf("wrong count of bytes read, got %d, exp %d", n, MustReadFileSize(f))
+	}
+}
 func TestWALWriter_WritePoints_Multiple(t *testing.T) {
 	dir := MustTempDir()
 	defer os.RemoveAll(dir)
@@ -490,7 +546,7 @@ func mustMarshalEntry(entry tsm1.WALEntry) (tsm1.WalEntryType, []byte) {
 
 	b, err := entry.Encode(bytes)
 	if err != nil {
-		panic("error encoding")
+		panic(fmt.Sprintf("error encoding: %v", err))
 	}
 
 	return entry.Type(), snappy.Encode(b, b)