response: { message: 'publisher is not defined', internalCode: undefined } }

I'm following codelab instructions to publish a topic in pubsub as below, but an error is returning:
response: { message: 'publisher is not defined', internalCode: undefined } }

at next (/home/google2145703_student/training-data-analyst/courses/developingapps/nodejs/pubsub-languageapi-spanner/start/node_modules/express/lib/router/index.js:275:10

Code:
// Handler for feedback POSTed from the client app
router.post('/feedback/:quiz', (req, res, next) => {
const feedback = req.body;
// TODO: Publish the message into Cloud Pub/Sub
publisher.publishFeedback(feedback).then(() => {
// TODO: Move the statement that returns a message to
// the client app here
res.json('Feedback received');

// END TODO

// TODO: Add a catch
}).catch(err => {
// TODO: There was an error, invoke the next middleware
next(err);

// END TODO
});

// END TODO
});