diff --git a/src/Address.php b/src/Address.php
index f371202..0df00c8 100644
--- a/src/Address.php
+++ b/src/Address.php
@@ -10,6 +10,8 @@
 
 namespace Pronamic\WordPress\Pay\Gateways\OmniKassa2;
 
+use JsonSerializable;
+
 /**
  * Address
  *
@@ -17,7 +19,7 @@
  * @version 2.1.10
  * @since   2.0.2
  */
-class Address implements \JsonSerializable {
+final class Address implements JsonSerializable {
 	/**
 	 * First name.
 	 *
diff --git a/src/Order.php b/src/Order.php
index 48a0ad2..09208dc 100644
--- a/src/Order.php
+++ b/src/Order.php
@@ -11,6 +11,7 @@
 namespace Pronamic\WordPress\Pay\Gateways\OmniKassa2;
 
 use DateTime;
+use JsonSerializable;
 
 /**
  * Order
@@ -19,7 +20,7 @@
  * @version 2.2.4
  * @since   1.0.0
  */
-class Order extends Message implements \JsonSerializable {
+final class Order extends Message implements JsonSerializable {
 	/**
 	 * ISO 8601 standard Date / time on which the order is announced at ROK.
 	 * As a rule, this is the current date / time.
diff --git a/src/OrderItem.php b/src/OrderItem.php
index 129f180..41ba35d 100644
--- a/src/OrderItem.php
+++ b/src/OrderItem.php
@@ -10,6 +10,8 @@
 
 namespace Pronamic\WordPress\Pay\Gateways\OmniKassa2;
 
+use JsonSerializable;
+
 /**
  * Order item.
  *
@@ -17,7 +19,7 @@
  * @version 2.1.10
  * @since   2.0.3
  */
-class OrderItem implements \JsonSerializable {
+final class OrderItem implements JsonSerializable {
 	/**
 	 * Item id.
 	 *