The library is not working

[vakavaka]

const express = require('express');
const { balancer, monitor } = require('proxy-supervisor');
const source = require('ps-nordvpn');
Tried this code

const freeBalancer = balancer()
    .subscribe(source)
    .subscribe(monitor);

const app = express(privateBalancer.proxy());
app.listen(3000);

Got error :
TypeError: source.addListener is not a function
at Balancer.subscribe (/home/valentin/projects/freelance/fake-users/proxy/node_modules/proxy-supervisor/lib/core/balancer.js:56:12)
at Object. (/home/valentin/projects/freelance/fake-users/proxy/run.js:6:6)
at Module._compile (internal/modules/cjs/loader.js:799:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:810:10)
at Module.load (internal/modules/cjs/loader.js:666:32)

Node v11.12.0, Linux, Ubuntu