Firebase Cloud Messaging не может прочитать свойство Receiver_id из undefined

У меня проблема с уведомлениями Firebase Cloud Messaging. Когда я хочу отправить запрос на добавление в друзья в своем приложении для Android, другой клиент не получает уведомления об этом. И журнал функций Firebase говорит:

TypeError: Cannot read property 'receiver_id' of undefined
    at exports.sendNotification.functions.database.ref.onWrite.event (/user_code/index.js:9:35)
    at cloudFunctionNewSignature (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:109:23)
    at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:139:20)
    at /var/tmp/worker/worker.js:730:24
    at process._tickDomainCallback (internal/process/next_tick.js:135:7)

Вот код JavaScript:

'use strict'

const functions = require('firebase-functions');
const admin = require ('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.sendNotification = functions.database.ref('/Notifications/{receiver_id}/{notification_id}')
.onWrite(event => {
  const receiver_id = event.params.receiver_id;

  const notification_id = event.params.notification_id;

  console.info('We have a notification to send to: ', receiver_id);

  if (!event.data.val) {
    return console.info('A notification has been deleted from database: ', notification_id);
  }

  const deviceToken = admin.database().ref(`/Users/${receiver_id}/device_token`).once('value');

  return deviceToken.then(result => {
    const token_id = result.val();

    const payload = {
      notification:
      {
        title: "Friend Request",
        body: "you have received a new friend request",
        icon: "default"
      }
    };

    return admin.messaging().sendToDevice(token_id, payload).then(response => {
      console.info('This was the notification feature.');
    });
  });

});