diff --git a/src/PrintNode/Entity.php b/src/PrintNode/Entity.php
index 1a71c51..2430020 100755
--- a/src/PrintNode/Entity.php
+++ b/src/PrintNode/Entity.php
@@ -102,7 +102,7 @@ public function mapValuesFromJson($json)
      * 
      * @return string
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         
         $json = array();
@@ -132,4 +132,4 @@ public function __toString() {
         
     }
     
-}
\ No newline at end of file
+}