diff --git a/src/controllers/newsletter.controller.js b/src/controllers/newsletter.controller.js
index dca5a7fe..8146a325 100644
--- a/src/controllers/newsletter.controller.js
+++ b/src/controllers/newsletter.controller.js
@@ -224,9 +224,15 @@ exports.listSubscriptions = async (req, res) => {
       ...(options.source && { source: options.source })
     });
 
+    // Convert ObjectId to string for JSON serialization
+    const serializedSubscriptions = subscriptions.map(sub => ({
+      ...sub,
+      _id: sub._id.toString()
+    }));
+
     res.json({
       success: true,
-      subscriptions,
+      subscriptions: serializedSubscriptions,
       pagination: {
         total,
         limit: options.limit,