diff --git a/klog.go b/klog.go
index 17d29752d..57889f892 100644
--- a/klog.go
+++ b/klog.go
@@ -746,6 +746,8 @@ func (rb *redirectBuffer) Write(bytes []byte) (n int, err error) {
 
 // SetOutput sets the output destination for all severities
 func SetOutput(w io.Writer) {
+	logging.mu.Lock()
+	defer logging.mu.Unlock()
 	for s := fatalLog; s >= infoLog; s-- {
 		rb := &redirectBuffer{
 			w: w,
@@ -756,6 +758,8 @@ func SetOutput(w io.Writer) {
 
 // SetOutputBySeverity sets the output destination for specific severity
 func SetOutputBySeverity(name string, w io.Writer) {
+	logging.mu.Lock()
+	defer logging.mu.Unlock()
 	sev, ok := severityByName(name)
 	if !ok {
 		panic(fmt.Sprintf("SetOutputBySeverity(%q): unrecognized severity name", name))