Heroku Logs - open log referred to in error message

I'm deploying a node.js app to heroku (cedar stack)

I've set the NODE_ENV like this:

heroku config:add NODE_ENV=production

I then do:

git push heroku

However, after starting (i've got a console.log generating the first log entry) the app crashes.

I run:

heroku logs

and get the below error.

How can I open the /app/log/production.log that it's referring to?

app[web.1]: listening on port 32168 within production environment
app[web.1]: 
app[web.1]: events.js:48
app[web.1]:         throw arguments[1]; // Unhandled 'error' event
app[web.1]:                        ^
app[web.1]: Error: ENOENT, open '/app/log/production.log'
heroku[web.1]: Process exited with status 1
heroku[web.1]: State changed from starting to crashed