diff --git a/Models/NullPromotion.php b/Models/NullPromotion.php
index baae8b5..9d5e30f 100755
--- a/Models/NullPromotion.php
+++ b/Models/NullPromotion.php
@@ -36,4 +36,12 @@ final class NullPromotion extends Promotion
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullPromotionAttribute.php b/Models/NullPromotionAttribute.php
index f09b0df..fde4a23 100755
--- a/Models/NullPromotionAttribute.php
+++ b/Models/NullPromotionAttribute.php
@@ -36,4 +36,12 @@ final class NullPromotionAttribute extends PromotionAttribute
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullPromotionAttributeType.php b/Models/NullPromotionAttributeType.php
index d1ef7f6..2ca95e2 100755
--- a/Models/NullPromotionAttributeType.php
+++ b/Models/NullPromotionAttributeType.php
@@ -35,4 +35,12 @@ final class NullPromotionAttributeType extends PromotionAttributeType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullPromotionAttributeTypeL11n.php b/Models/NullPromotionAttributeTypeL11n.php
index e0853cd..411c1bb 100755
--- a/Models/NullPromotionAttributeTypeL11n.php
+++ b/Models/NullPromotionAttributeTypeL11n.php
@@ -35,4 +35,12 @@ final class NullPromotionAttributeTypeL11n extends PromotionAttributeTypeL11n
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullPromotionAttributeValue.php b/Models/NullPromotionAttributeValue.php
index 535de3d..3a1b2ae 100755
--- a/Models/NullPromotionAttributeValue.php
+++ b/Models/NullPromotionAttributeValue.php
@@ -35,4 +35,12 @@ final class NullPromotionAttributeValue extends PromotionAttributeValue
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }