Reproduce issue with postcss-modules and cssnano

package.json

{
  "name": "postcss-bug",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "cssnano": "^3.5.2",
    "postcss": "^5.0.19",
    "postcss-modules": "^0.4.0"
  }
}

process.js

const postcss = require('postcss');
const cssnano = require('cssnano');
const modules = require('postcss-modules');

process.on('unhandledRejection', e => {
    console.log(e.stack);
});

postcss([
    cssnano(),
    modules({
        getJSON() {}
    })
]).process('.a {color: white}')
    .then(
        result => console.log(result.css),
        e => console.error(e.stack)
    );