Custom configuration entry in environment.js

I use EmberJS 2.4.2. I want to use environment.js file to handle custom configuration entries. E.g.

var ENV = {
  APP: {
    myKey: "defaultValue"
  }
};

It works fine in development mode, but after I perform 'ember build -prod', the variable is inaccessible - in the production mode the variable is 'undefined'.

I use the key in following manner:

import ENV from '../config/environment';
(...)
ENV.APP.myKey

What am I doing wrong and how to resolve the problem correctly?