diff --git a/src/Message.php b/src/Message.php
index d2627b4..7752c07 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -40,7 +40,7 @@ public function key(): string
         return $this->key;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return get_object_vars($this);
     }