diff --git a/src/CountryLoader.php b/src/CountryLoader.php
index 115e43d..61a1cb6 100644
--- a/src/CountryLoader.php
+++ b/src/CountryLoader.php
@@ -122,7 +122,7 @@ protected static function operatorForWhere($key, $operator, $value)
      *
      * @return array
      */
-    protected static function filter($items, callable $callback = null)
+    protected static function filter($items, ?callable $callback = null)
     {
         if ($callback) {
             return array_filter($items, $callback, ARRAY_FILTER_USE_BOTH);