diff --git a/src/Symfony/Component/Notifier/Message/ChatMessage.php b/src/Symfony/Component/Notifier/Message/ChatMessage.php
index f9b0b280c9f48..5fc1d94a3a322 100644
--- a/src/Symfony/Component/Notifier/Message/ChatMessage.php
+++ b/src/Symfony/Component/Notifier/Message/ChatMessage.php
@@ -77,7 +77,7 @@ public function getOptions(): ?MessageOptionsInterface
     /**
      * @return $this
      */
-    public function transport(string $transport): self
+    public function transport(?string $transport): self
     {
         $this->transport = $transport;