I'm having issues getting data from an API while learning Angular 2. This is the error I'm getting

url.js:106 throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'url', 'string', url);
TypeError [ERR_INVALID_ARG_TYPE]: The url argument must be of type string. Received type undefined at Url.parse (url.js:106:11)

This is the code I'm working with

index.js

const express = require('express');

const app = express();



const MongoClient = require('mongodb').MongoClient;



require('dotenv').config();



let database;



MongoClient.connect(process.env.DB_CONN, (err, db) => {



console.log('connected to mongodb...');



app.listen(3000, () => {

    database = db;

    console.log('listening on port 3000...')

});

});



app.get('/contacts', (req, res) => {

const contactsCollection = database.collection('contacts');



contactsCollection.find({}).toArray((err, docs) => {

    if(err) {

        console.log(err);

    }

    return res.json(docs);

});

});

.env

DB_CONN=mongodb://admin:[email protected]:17931/dem-contact

I do not understand the error and the only url I have is in my env file since I'm using mlab mongodb. What's wrong with my code?