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"
  }
}