diff --git a/src/DataType/InstantPaymentNotification.php b/src/DataType/InstantPaymentNotification.php
index 1f466fb..26e3a7c 100644
--- a/src/DataType/InstantPaymentNotification.php
+++ b/src/DataType/InstantPaymentNotification.php
@@ -47,7 +47,7 @@ class InstantPaymentNotification extends ResponseBase implements JsonSerializabl
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $data = [];
 
diff --git a/src/DataType/PaymentRequest.php b/src/DataType/PaymentRequest.php
index adbb60a..33c6b50 100644
--- a/src/DataType/PaymentRequest.php
+++ b/src/DataType/PaymentRequest.php
@@ -128,7 +128,7 @@ class PaymentRequest extends RequestBase
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         $data = [];