diff --git a/Models/NullKanbanBoard.php b/Models/NullKanbanBoard.php
index 8a8fcd4..56f15fb 100755
--- a/Models/NullKanbanBoard.php
+++ b/Models/NullKanbanBoard.php
@@ -36,4 +36,12 @@ final class NullKanbanBoard extends KanbanBoard
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullKanbanCard.php b/Models/NullKanbanCard.php
index a278719..5461588 100755
--- a/Models/NullKanbanCard.php
+++ b/Models/NullKanbanCard.php
@@ -36,4 +36,12 @@ final class NullKanbanCard extends KanbanCard
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullKanbanCardComment.php b/Models/NullKanbanCardComment.php
index 05ac8b6..20c44a5 100755
--- a/Models/NullKanbanCardComment.php
+++ b/Models/NullKanbanCardComment.php
@@ -36,4 +36,12 @@ final class NullKanbanCardComment extends KanbanCardComment
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullKanbanColumn.php b/Models/NullKanbanColumn.php
index ca32645..f1b050f 100755
--- a/Models/NullKanbanColumn.php
+++ b/Models/NullKanbanColumn.php
@@ -35,4 +35,12 @@ final class NullKanbanColumn extends KanbanColumn
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }