diff --git a/Models/NullWorkflowInstanceAbstract.php b/Models/NullWorkflowInstanceAbstract.php
index a989326..4371f6f 100755
--- a/Models/NullWorkflowInstanceAbstract.php
+++ b/Models/NullWorkflowInstanceAbstract.php
@@ -35,4 +35,12 @@ final class NullWorkflowInstanceAbstract extends WorkflowInstanceAbstract
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullWorkflowTemplate.php b/Models/NullWorkflowTemplate.php
index 53869d2..2c02d89 100755
--- a/Models/NullWorkflowTemplate.php
+++ b/Models/NullWorkflowTemplate.php
@@ -36,4 +36,12 @@ final class NullWorkflowTemplate extends WorkflowTemplate
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }