diff --git a/functions/android.js b/functions/android.js
index 160f511..6d7ed56 100644
--- a/functions/android.js
+++ b/functions/android.js
@@ -113,7 +113,12 @@ module.exports = {
 
       androidNotificationKeys.forEach((key) => {
         if (Object.hasOwn(req.body.data, key)) {
-          payload.data[key] = String(req.body.data[key]);
+          const v = req.body.data[key];
+          // FCM data values must be strings. For arrays/objects (e.g. when the
+          // calling integration's template engine auto-parses a JSON-string
+          // back into a list), String(value) produces "[object Object]" which
+          // the receiving app then can't decode as JSON. Stringify properly.
+          payload.data[key] = (v !== null && typeof v === 'object') ? JSON.stringify(v) : String(v);
         }
       });
     }
diff --git a/functions/handlers.js b/functions/handlers.js
index adf4677..5dc50ae 100644
--- a/functions/handlers.js
+++ b/functions/handlers.js
@@ -168,8 +168,11 @@ function handleError(req, res, payload = {}, step, incomingError, shouldExit = t
     incomingError = new Error(incomingError);
   }
 
-  // Handle Firebase Messaging errors with appropriate status codes
-  if (incomingError.code?.startsWith('messaging/')) {
+  // Handle Firebase Messaging errors with appropriate status codes.
+  // `code` can be a non-string (e.g. numeric HTTP status from a network failure
+  // before reaching FCM); without the type guard, .startsWith throws and masks
+  // the underlying error.
+  if (typeof incomingError.code === 'string' && incomingError.code.startsWith('messaging/')) {
     const errorCode = incomingError.code.replace('messaging/', '');
 
     // For specific token errors, skip reporting and return immediately
diff --git a/functions/test/android.test.js b/functions/test/android.test.js
new file mode 100644
index 0000000..577b536
--- /dev/null
+++ b/functions/test/android.test.js
@@ -0,0 +1,50 @@
+'use strict';
+
+const android = require('../android.js');
+
+describe('android.js createPayload', () => {
+  function makeReq(data) {
+    return {
+      body: {
+        data,
+        registration_info: {},
+      },
+    };
+  }
+
+  describe('whitelisted data values', () => {
+    test('primitive values are stringified', () => {
+      const { payload } = android.createPayload(makeReq({ tag: 'foo', importance: 4 }));
+      expect(payload.data.tag).toBe('foo');
+      expect(payload.data.importance).toBe('4');
+    });
+
+    test('array values are JSON-stringified, not coerced via String()', () => {
+      // Repro: HA's template engine auto-parses a JSON array string back into a
+      // native list before posting. Without JSON.stringify, the proxy ships
+      // "1,2,3" (Array.prototype.toString), and the receiving app's JSON.parse
+      // throws SyntaxError. Object array would arrive as "[object Object]".
+      const samples = [
+        { time: '2026-01-01T08:00:00Z', value: 60 },
+        { time: '2026-01-01T08:00:30Z', value: 64 },
+      ];
+      const { payload } = android.createPayload(makeReq({ vibrationPattern: samples }));
+      expect(payload.data.vibrationPattern).toBe(JSON.stringify(samples));
+      expect(JSON.parse(payload.data.vibrationPattern)).toEqual(samples);
+    });
+
+    test('object values are JSON-stringified', () => {
+      const obj = { a: 1, b: 'two' };
+      const { payload } = android.createPayload(makeReq({ intent_extras: obj }));
+      expect(payload.data.intent_extras).toBe(JSON.stringify(obj));
+    });
+
+    test('null values fall through to String() and become "null"', () => {
+      // Documents existing behavior — JSON.stringify(null) is also 'null', but
+      // explicitly guarding `typeof v === 'object' && v !== null` keeps null
+      // out of the JSON branch so this stays consistent with prior versions.
+      const { payload } = android.createPayload(makeReq({ tag: null }));
+      expect(payload.data.tag).toBe('null');
+    });
+  });
+});
diff --git a/functions/test/fcm-errors.test.js b/functions/test/fcm-errors.test.js
index 1a3a9d7..71490a3 100644
--- a/functions/test/fcm-errors.test.js
+++ b/functions/test/fcm-errors.test.js
@@ -196,4 +196,50 @@ describe('FCM Error Handling', () => {
     expect(mockLogging.log).toHaveBeenCalledWith('errors-sendNotification');
     expect(mockLogInstance.write).toHaveBeenCalled();
   });
+
+  test('should not crash when error.code is a non-string (numeric HTTP status)', async () => {
+    // Repro: a network failure before reaching FCM produces an Error with a numeric
+    // `code` (e.g. 503). Without the type guard in handleError, calling
+    // .startsWith('messaging/') on the number throws a TypeError that escapes
+    // handleError itself and the response is never sent.
+    const error = new Error('Service unavailable');
+    error.code = 503;
+    mockMessaging.send.mockRejectedValue(error);
+
+    const mockLogInstance = {
+      write: jest.fn((entry, callback) => callback()),
+      entry: jest.fn(() => ({})),
+    };
+    mockLogging.log.mockReturnValue(mockLogInstance);
+
+    await indexModule.handleRequest(req, res, payloadHandler);
+
+    expect(res.status).toHaveBeenCalledWith(500);
+    expect(res.send).toHaveBeenCalledWith({
+      errorType: 'InternalError',
+      errorStep: 'sendNotification',
+      message: 'Service unavailable',
+    });
+    expect(mockLogInstance.write).toHaveBeenCalled();
+  });
+
+  test('should not crash when error.code is undefined', async () => {
+    const error = new Error('Generic failure with no code');
+    mockMessaging.send.mockRejectedValue(error);
+
+    const mockLogInstance = {
+      write: jest.fn((entry, callback) => callback()),
+      entry: jest.fn(() => ({})),
+    };
+    mockLogging.log.mockReturnValue(mockLogInstance);
+
+    await indexModule.handleRequest(req, res, payloadHandler);
+
+    expect(res.status).toHaveBeenCalledWith(500);
+    expect(res.send).toHaveBeenCalledWith({
+      errorType: 'InternalError',
+      errorStep: 'sendNotification',
+      message: 'Generic failure with no code',
+    });
+  });
 });