Heroku node app running error code=H10 despite using `process.env.PORT`

Question

From what I learned from the documentation this error is because of a wrong port, but I use process.env.PORT in stead of localhost:3000 where I am running it locally and it still doesn't work.

Here is my code:

module.exports = env => {

let CLIENT_PORT;
let SERVER_PORT;
let CLIENT_LOG_LEVEL;
let BUILD_DIRECTORY_PREFIX;

if (env) {
    //CLIENT_PORT = env.CLIENT_PORT;
    //SERVER_PORT = env.SERVER_PORT;
    CLIENT_PORT = process.env.PORT || 3000
    SERVER_PORT = process.env.PORT || 3000
    CLIENT_LOG_LEVEL = JSON.stringify(env.CLIENT_LOG_LEVEL || 'ERROR');
    BUILD_DIRECTORY_PREFIX = env.BUILD_DIRECTORY_PREFIX;
}

return {
    entry: { app: './src/entry.js' },
    devServer: { hot: true, open: true, port: CLIENT_PORT },
    module: MODULE_RULES,
    output: { filename: "bundle.js", path: path.resolve(BUILD_DIRECTORY_PREFIX, './client/dist/public/') },
    performance: { hints: false },
    plugins: [
        new CleanWebpackPlugin(),
        new webpack.DefinePlugin({'process.env.SERVER_PORT': SERVER_PORT}),
        new webpack.DefinePlugin({'process.env.CLIENT_LOG_LEVEL': CLIENT_LOG_LEVEL}),
        new HtmlWebpackPlugin({ template: 'templates/index.html', favicon: "templates/favicon.ico" }),
        new webpack.HotModuleReplacementPlugin()
    ]
};
 };

I tried both running process.env.PORT || 3000 and process.env.PORT || 80 but nothing works. Any ideas what might be the problem?