diff --git a/Models/NullContract.php b/Models/NullContract.php
index 93694fb..3f81f8c 100755
--- a/Models/NullContract.php
+++ b/Models/NullContract.php
@@ -36,4 +36,12 @@ final class NullContract extends Contract
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullContractType.php b/Models/NullContractType.php
index 9a75955..36a652d 100755
--- a/Models/NullContractType.php
+++ b/Models/NullContractType.php
@@ -36,4 +36,12 @@ final class NullContractType extends ContractType
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullContractTypeL11n.php b/Models/NullContractTypeL11n.php
index d232979..4e42cb9 100755
--- a/Models/NullContractTypeL11n.php
+++ b/Models/NullContractTypeL11n.php
@@ -36,4 +36,12 @@ final class NullContractTypeL11n extends ContractTypeL11n
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }