diff --git a/src/Timer.php b/src/Timer.php
index 3398b40e2..c11db9d54 100644
--- a/src/Timer.php
+++ b/src/Timer.php
@@ -78,7 +78,7 @@ class Timer
      * @param EventInterface|null $event
      * @return void
      */
-    public static function init(EventInterface $event = null): void
+    public static function init(?EventInterface $event = null): void
     {
         if ($event) {
             self::$event = $event;