cuando quiero actualizar Cloud Firestore desde Realtime Database, implementé el siguiente código y recibo un error.

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

const firestore = functions.firestore;

exports.onUserStatusChange = functions.database
    .ref('/status/{userId}')
    .onUpdate(event => {

        var db = admin.firestore();


        //const usersRef = firestore.document('/users/' + event.params.userId);
        const usersRef = db.collection("users");
        var snapShot = event.data;

        return event.data.ref.once('value')
            .then(statusSnap => snapShot.val())
            .then(status => {
                if (status === 'offline'){
                    usersRef
                        .doc(event.params.userId)
                        .set({
                            online: false,
                            last_active: Date.now()
                        }, {merge: true});
                }
            })
    });

TypeError: no se puede leer la propiedad 'ref' de undefined en exports.onUserStatusChange.functions.database.ref.onUpdate.event (/user_code/index.js:18:20) en cloudFunctionNewSignature (/ user_code / node_modules / firebase-functions / lib /cloud-functions.js:105:23) en cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:135:20) en /var/tmp/worker/worker.js:733:24 en process._tickDomainCallback (internal / process / next_tick.js: 135: 7)