diff --git a/internal/ztest/writer.go b/internal/ztest/writer.go
index 9fdd5805e..b9a1928dc 100644
--- a/internal/ztest/writer.go
+++ b/internal/ztest/writer.go
@@ -25,6 +25,7 @@ import (
 	"errors"
 	"io/ioutil"
 	"strings"
+	"sync"
 )
 
 // A Syncer is a spy for the Sync portion of zapcore.WriteSyncer.
@@ -94,3 +95,28 @@ func (b *Buffer) Lines() []string {
 func (b *Buffer) Stripped() string {
 	return strings.TrimRight(b.String(), "\n")
 }
+
+// SyncBuffer is an implementation of bytes.Buffer which is goroutine safe.
+type SyncBuffer struct {
+	sync.RWMutex
+
+	buf bytes.Buffer
+}
+
+// Write appends the contents of p to the buffer, growing the buffer as
+// needed.
+func (b *SyncBuffer) Write(p []byte) (n int, err error) {
+	b.Lock()
+	defer b.Unlock()
+
+	return b.buf.Write(p)
+}
+
+// String returns the contents of the unread portion of the buffer
+// as a string.
+func (b *SyncBuffer) String() string {
+	b.RLock()
+	defer b.RUnlock()
+
+	return b.buf.String()
+}
diff --git a/zapcore/write_syncer_test.go b/zapcore/write_syncer_test.go
index 98d62ff75..e41ae2369 100644
--- a/zapcore/write_syncer_test.go
+++ b/zapcore/write_syncer_test.go
@@ -122,7 +122,7 @@ func TestBufferWriter(t *testing.T) {
 	})
 
 	t.Run("flush timer", func(t *testing.T) {
-		buf := &bytes.Buffer{}
+		buf := &ztest.SyncBuffer{}
 		ws, close := Buffer(AddSync(buf), 6, time.Microsecond)
 		defer close()
 		requireWriteWorks(t, ws)