diff --git a/service.go b/service.go
index 503215f..8aac8fd 100644
--- a/service.go
+++ b/service.go
@@ -40,7 +40,7 @@ type Option func(*Service)
 
 // WithFallback sets the fallback language for missing translations.
 func WithFallback(lang string) Option {
-	return func(s *Service) { s.fallbackLang = lang }
+	return func(s *Service) { s.fallbackLang = normalizeLanguageTag(lang) }
 }
 
 // WithFormality sets the default formality level.
@@ -290,7 +290,7 @@ func (s *Service) SetMode(m Mode)           { s.mu.Lock(); s.mode = m; s.mu.Unlo
 func (s *Service) Mode() Mode               { s.mu.RLock(); defer s.mu.RUnlock(); return s.mode }
 func (s *Service) SetFormality(f Formality) { s.mu.Lock(); s.formality = f; s.mu.Unlock() }
 func (s *Service) Formality() Formality     { s.mu.RLock(); defer s.mu.RUnlock(); return s.formality }
-func (s *Service) SetFallback(lang string)  { s.mu.Lock(); s.fallbackLang = lang; s.mu.Unlock() }
+func (s *Service) SetFallback(lang string)  { s.mu.Lock(); s.fallbackLang = normalizeLanguageTag(lang); s.mu.Unlock() }
 func (s *Service) Fallback() string {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
diff --git a/service_test.go b/service_test.go
index 9d34e01..8c41676 100644
--- a/service_test.go
+++ b/service_test.go
@@ -370,6 +370,22 @@ func TestServiceFallback(t *testing.T) {
 	}
 }
 
+func TestServiceFallbackNormalisesLanguageTag(t *testing.T) {
+	svc, err := New(WithFallback("fr_CA"))
+	if err != nil {
+		t.Fatalf("New() failed: %v", err)
+	}
+
+	if got, want := svc.Fallback(), "fr-CA"; got != want {
+		t.Fatalf("WithFallback(fr_CA) = %q, want %q", got, want)
+	}
+
+	svc.SetFallback("en_US")
+	if got, want := svc.Fallback(), "en-US"; got != want {
+		t.Fatalf("SetFallback(en_US) = %q, want %q", got, want)
+	}
+}
+
 func TestServiceMessageFallbackUsesBaseLanguageTagBeforeConfiguredFallback(t *testing.T) {
 	svc, err := NewWithLoader(messageBaseFallbackLoader{})
 	if err != nil {