diff --git a/src/PushTemplate.php b/src/PushTemplate.php
index 1a76e90..49d31a6 100644
--- a/src/PushTemplate.php
+++ b/src/PushTemplate.php
@@ -297,7 +297,7 @@ class PushTemplate implements \JsonSerializable
         ];
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return $this->toArray();
     }