diff --git a/hooks.go b/hooks.go
index 168c134..a19a47c 100644
--- a/hooks.go
+++ b/hooks.go
@@ -11,6 +11,7 @@ import (
 )
 
 var missingKeyHandler atomic.Value
+var missingKeyHandlerMu sync.Mutex
 
 type missingKeyHandlersState struct {
 	handlers []MissingKeyHandler
@@ -131,6 +132,8 @@ func loadLocaleProvider(svc *Service, provider localeProviderRegistration) {
 
 // OnMissingKey registers a handler for missing translation keys.
 func OnMissingKey(h MissingKeyHandler) {
+	missingKeyHandlerMu.Lock()
+	defer missingKeyHandlerMu.Unlock()
 	if h == nil {
 		missingKeyHandler.Store(missingKeyHandlersState{})
 		return
@@ -140,6 +143,8 @@ func OnMissingKey(h MissingKeyHandler) {
 
 // ClearMissingKeyHandlers removes all registered missing-key handlers.
 func ClearMissingKeyHandlers() {
+	missingKeyHandlerMu.Lock()
+	defer missingKeyHandlerMu.Unlock()
 	missingKeyHandler.Store(missingKeyHandlersState{})
 }
 
@@ -149,6 +154,8 @@ func AddMissingKeyHandler(h MissingKeyHandler) {
 	if h == nil {
 		return
 	}
+	missingKeyHandlerMu.Lock()
+	defer missingKeyHandlerMu.Unlock()
 	current := missingKeyHandlers()
 	current.handlers = append(current.handlers, h)
 	missingKeyHandler.Store(current)
diff --git a/hooks_test.go b/hooks_test.go
index e9114f4..fa829d3 100644
--- a/hooks_test.go
+++ b/hooks_test.go
@@ -2,6 +2,7 @@ package i18n
 
 import (
 	"path/filepath"
+	"sync"
 	"testing"
 	"testing/fstest"
 
@@ -456,6 +457,38 @@ func TestAddMissingKeyHandler_Good(t *testing.T) {
 	assert.Equal(t, 1, second)
 }
 
+func TestAddMissingKeyHandler_Good_Concurrent(t *testing.T) {
+	svc, err := New()
+	require.NoError(t, err)
+	prev := Default()
+	SetDefault(svc)
+	prevHandlers := missingKeyHandlers()
+	t.Cleanup(func() {
+		missingKeyHandler.Store(prevHandlers)
+		SetDefault(prev)
+	})
+	svc.SetMode(ModeCollect)
+
+	ClearMissingKeyHandlers()
+	t.Cleanup(func() {
+		ClearMissingKeyHandlers()
+	})
+
+	const handlers = 32
+	var wg sync.WaitGroup
+	wg.Add(handlers)
+	for i := 0; i < handlers; i++ {
+		go func() {
+			defer wg.Done()
+			AddMissingKeyHandler(func(MissingKey) {})
+		}()
+	}
+	wg.Wait()
+
+	state := missingKeyHandlers()
+	assert.Len(t, state.handlers, handlers)
+}
+
 func TestClearMissingKeyHandlers_Good(t *testing.T) {
 	svc, err := New()
 	require.NoError(t, err)