diff --git a/CHANGELOG.md b/CHANGELOG.md
index a8cd13d..0da3ac6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project follows [Semantic Versioning](https://semver.org/).
 
 ## [Unreleased]
 
+### Fixed
+
+- "Add to calendar" (`GET /api/events/:id/ics`) always failed with a 400 in production: the `ics` library's `url` field requires a scheme (`https://...`), but `FRONTEND_URL`/`APP_BASE_URL` was configured without one, so `createEvent()` rejected it before generating any output. A missing scheme is now assumed to be `https://` instead of failing.
+
 ## [1.10.3] - 2026-08-28
 
 ### Fixed
diff --git a/backend/src/controllers/eventController.js b/backend/src/controllers/eventController.js
index ebce80e..964c32b 100644
--- a/backend/src/controllers/eventController.js
+++ b/backend/src/controllers/eventController.js
@@ -493,7 +493,12 @@ const getEventIcs = async (req, res) => {
     }
 
     const { buildEventIcs } = require('../utils/icsUtils');
-    const frontendUrl = (process.env.FRONTEND_URL || process.env.APP_BASE_URL || 'http://localhost:3001').replace(/\/$/, '');
+    let frontendUrl = (process.env.FRONTEND_URL || process.env.APP_BASE_URL || 'http://localhost:3001').replace(/\/$/, '');
+    // The ics library's url validator requires a scheme; guard against FRONTEND_URL/APP_BASE_URL
+    // being configured without one (e.g. "example.com" instead of "https://example.com").
+    if (!/^[a-z0-9+.-]+:\/\//i.test(frontendUrl)) {
+      frontendUrl = `https://${frontendUrl}`;
+    }
     const ics = buildEventIcs(event, `${frontendUrl}/events/${event.id}`);
 
     res.setHeader('Content-Type', 'text/calendar; charset=utf-8');