read-only config values using nconf and env

I'm using nconf to handle configuration in my app. The way I'm setting it up is as follows:

nconf.env({
    separator: '__',
    whitelist: ['foo', 'bar']
})
.file('config.json')

It appears that I'm not able to modify values if they were obtained via an environment variable. For example,

console.log(nconf.get()); // {"foo":123,"bar":356}
nconf.set('foo', 789);
console.log(nconf.get()); // {"foo":123,"bar":356}

I have inspected the stores attribute of nconf and it seems to suggest that env variables are read-only?

console.log(nconf.stores);
/**
 * { env:
 *   { type: 'env',
 *     store: { foo: [Object] },
 *     mtimes: { 'foo': 1372348332705 },
 *     readOnly: true, <-- here
 *     loadFrom: null,
 *     whitelist:
 *         ...

Is there a way to allow variables set via env variables to be modified at runtime? If I set values which were set using the config.json file I am able to modify values without any problems.

Any help much appreciated :-)