diff --git a/Part/MessagePart.php b/Part/MessagePart.php
index 00129b4..270d57a 100644
--- a/Part/MessagePart.php
+++ b/Part/MessagePart.php
@@ -60,10 +60,7 @@ public function bodyToIterable(): iterable
         return $this->message->toIterable();
     }
 
-    /**
-     * @return array
-     */
-    public function __sleep()
+    public function __sleep(): array
     {
         return ['message'];
     }