diff --git a/src/MultilingualRoutePendingRegistration.php b/src/MultilingualRoutePendingRegistration.php
index 729acd2..974b8f2 100644
--- a/src/MultilingualRoutePendingRegistration.php
+++ b/src/MultilingualRoutePendingRegistration.php
@@ -212,7 +212,7 @@ public function only($locales): self
      * @param  string|null  $locale
      * @return $this
      */
-    public function where($name, ?string $expression = null, string $locale = null): self
+    public function where($name, ?string $expression = null, ?string $locale = null): self
     {
         $key = rtrim("constraints-$locale", '-');