diff --git a/src/Websocket/Pusher.php b/src/Websocket/Pusher.php
index 32b8e91..a5cdc87 100644
--- a/src/Websocket/Pusher.php
+++ b/src/Websocket/Pusher.php
@@ -68,7 +68,7 @@ protected function __construct(
         bool $broadcast,
         bool $assigned,
         string $event,
-        $message = null,
+        ?string $message,
         $server
     )
     {