Why am I getting "Configuration property is not defined"?

When I run my index.js file, I am getting Configuration property "mail.host" is not defined. I don't understand why.

config.get('name') works fine. I set my NODE_ENV=production. Can anyone please help?

index.js

const config = require('config');console.log(config.get('name'));console.log(config.get('mail.host'));

production.json

{"name": "My app - production","mail": {"host": "production-server"  }}