diff --git a/Models/NullCalendar.php b/Models/NullCalendar.php
index d4e9dbd..355e476 100755
--- a/Models/NullCalendar.php
+++ b/Models/NullCalendar.php
@@ -36,4 +36,12 @@ final class NullCalendar extends Calendar
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEvent.php b/Models/NullEvent.php
index 664afc6..993abc0 100755
--- a/Models/NullEvent.php
+++ b/Models/NullEvent.php
@@ -36,4 +36,12 @@ final class NullEvent extends Event
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullSchedule.php b/Models/NullSchedule.php
index faec684..0c139d9 100755
--- a/Models/NullSchedule.php
+++ b/Models/NullSchedule.php
@@ -36,4 +36,12 @@ final class NullSchedule extends Schedule
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }