diff --git a/Models/NullCostCenter.php b/Models/NullCostCenter.php
index 10d3a43..e72080b 100755
--- a/Models/NullCostCenter.php
+++ b/Models/NullCostCenter.php
@@ -35,4 +35,12 @@ final class NullCostCenter extends CostCenter
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullCostObject.php b/Models/NullCostObject.php
index a0b8221..7de371d 100755
--- a/Models/NullCostObject.php
+++ b/Models/NullCostObject.php
@@ -35,4 +35,12 @@ final class NullCostObject extends CostObject
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }