diff --git a/Models/NullBill.php b/Models/NullBill.php
index 89a5b22..0fce6ea 100755
--- a/Models/NullBill.php
+++ b/Models/NullBill.php
@@ -36,4 +36,12 @@ final class NullBill extends Bill
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullBillElement.php b/Models/NullBillElement.php
index a7ad65e..77a5719 100755
--- a/Models/NullBillElement.php
+++ b/Models/NullBillElement.php
@@ -36,4 +36,12 @@ final class NullBillElement extends BillElement
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullBillType.php b/Models/NullBillType.php
index 1cba56c..2e366b8 100755
--- a/Models/NullBillType.php
+++ b/Models/NullBillType.php
@@ -36,4 +36,12 @@ final class NullBillType extends BillType
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }