diff --git a/src/Annotations/Operation.php b/src/Annotations/Operation.php
index bc089dc1..7f926865 100644
--- a/src/Annotations/Operation.php
+++ b/src/Annotations/Operation.php
@@ -194,7 +194,7 @@ abstract class Operation extends AbstractAnnotation
      * @inheritdoc
      */
     #[\ReturnTypeWillChange]
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $data = parent::jsonSerialize();