MongoError: not authorized on admin to execute command.

// build mongo database connection url //

process.env.DB_HOST = process.env.DB_HOST || 'localhost'
process.env.DB_PORT = process.env.DB_PORT || 27017;
process.env.DB_NAME = process.env.DB_NAME || 'node-login';

if (app.get('env') != 'live'){
    process.env.DB_URL = 'mongodb://'+process.env.DB_HOST+':'+process.env.DB_PORT;
}   else {
// prepend url with authentication credentials //
    process.env.DB_URL = 'mongodb://'+process.env.DB_USER+':'+process.env.DB_PASS+'@'+process.env.DB_HOST+':'+process.env.DB_PORT;
}

app.use(session({
    secret: 'faeb4453e5d14fe6f6d04637f78077c76c73d1b4',
    proxy: true,
    resave: true,
    saveUninitialized: true,
    store: new MongoStore({ url: process.env.DB_URL })
    })
);