diff --git a/Models/NullEventAttribute.php b/Models/NullEventAttribute.php
index 1b20bc4..90f4061 100755
--- a/Models/NullEventAttribute.php
+++ b/Models/NullEventAttribute.php
@@ -36,4 +36,12 @@ final class NullEventAttribute extends EventAttribute
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEventAttributeType.php b/Models/NullEventAttributeType.php
index fc2069b..351ef5c 100755
--- a/Models/NullEventAttributeType.php
+++ b/Models/NullEventAttributeType.php
@@ -35,4 +35,12 @@ final class NullEventAttributeType extends EventAttributeType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEventAttributeTypeL11n.php b/Models/NullEventAttributeTypeL11n.php
index e7c4f9a..72a7182 100755
--- a/Models/NullEventAttributeTypeL11n.php
+++ b/Models/NullEventAttributeTypeL11n.php
@@ -35,4 +35,12 @@ final class NullEventAttributeTypeL11n extends EventAttributeTypeL11n
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEventAttributeValue.php b/Models/NullEventAttributeValue.php
index 2b79639..52af573 100755
--- a/Models/NullEventAttributeValue.php
+++ b/Models/NullEventAttributeValue.php
@@ -35,4 +35,12 @@ final class NullEventAttributeValue extends EventAttributeValue
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }