diff --git a/pkg/oidc/types.go b/pkg/oidc/types.go
index faf8e7f..0e7152c 100644
--- a/pkg/oidc/types.go
+++ b/pkg/oidc/types.go
@@ -77,6 +77,10 @@ func (l *Locale) MarshalJSON() ([]byte, error) {
 	return json.Marshal(tag)
 }
 
+// UnmarshalJSON implements json.Unmarshaler.
+// When [language.ValueError] is encountered, the containing tag will be set
+// to an empty value (language "und") and no error will be returned.
+// This state can be checked with the `l.Tag().IsRoot()` method.
 func (l *Locale) UnmarshalJSON(data []byte) error {
 	err := json.Unmarshal(data, &l.tag)
 	if err == nil {
@@ -86,6 +90,7 @@ func (l *Locale) UnmarshalJSON(data []byte) error {
 	// catch "well-formed but unknown" errors
 	var target language.ValueError
 	if errors.As(err, &target) {
+		l.tag = language.Tag{}
 		return nil
 	}
 	return err
diff --git a/pkg/oidc/userinfo.go b/pkg/oidc/userinfo.go
index b824160..ef8ebe4 100644
--- a/pkg/oidc/userinfo.go
+++ b/pkg/oidc/userinfo.go
@@ -41,13 +41,7 @@ func (u *UserInfo) MarshalJSON() ([]byte, error) {
 }
 
 func (u *UserInfo) UnmarshalJSON(data []byte) error {
-	if err := unmarshalJSONMulti(data, (*uiAlias)(u), &u.Claims); err != nil {
-		return err
-	}
-	if u.Locale != nil && u.Locale.tag.IsRoot() {
-		u.Locale = nil
-	}
-	return nil
+	return unmarshalJSONMulti(data, (*uiAlias)(u), &u.Claims)
 }
 
 type UserInfoProfile struct {