diff --git a/Models/NullClient.php b/Models/NullClient.php
index 080c417..a8df3f9 100755
--- a/Models/NullClient.php
+++ b/Models/NullClient.php
@@ -36,4 +36,12 @@ final class NullClient extends Client
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullClientAttribute.php b/Models/NullClientAttribute.php
index 58a310d..dae1ea7 100755
--- a/Models/NullClientAttribute.php
+++ b/Models/NullClientAttribute.php
@@ -36,4 +36,12 @@ final class NullClientAttribute extends ClientAttribute
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullClientAttributeType.php b/Models/NullClientAttributeType.php
index 9930318..7ff5fe5 100755
--- a/Models/NullClientAttributeType.php
+++ b/Models/NullClientAttributeType.php
@@ -35,4 +35,12 @@ final class NullClientAttributeType extends ClientAttributeType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullClientAttributeTypeL11n.php b/Models/NullClientAttributeTypeL11n.php
index 45641ce..f500660 100755
--- a/Models/NullClientAttributeTypeL11n.php
+++ b/Models/NullClientAttributeTypeL11n.php
@@ -35,4 +35,12 @@ final class NullClientAttributeTypeL11n extends ClientAttributeTypeL11n
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullClientAttributeValue.php b/Models/NullClientAttributeValue.php
index dc6dc86..5c69363 100755
--- a/Models/NullClientAttributeValue.php
+++ b/Models/NullClientAttributeValue.php
@@ -35,4 +35,12 @@ final class NullClientAttributeValue extends ClientAttributeValue
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }