Cannot read property 'listen' of undefined

Estou tendo o seguinte problema ao ligar o node:

C:\Users\Usuario\payfast>node index.js
consign v0.1.2 Initialized in C:\Users\Usuario\payfast
+ .\controllers\pagamento.js
C:\Users\Usuario\payfast\index.js:3
app.listen(3000, function(){
   ^

TypeError: Cannot read property 'listen' of undefined
    at Object.<anonymous> (C:\Users\Usuario\payfast\index.js:3:4)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:394:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:509:3

C:\Users\Usuario\payfast>

Meu código esta assim:

Custom-express

var express = require('express');
  var consign = require('consign');

  var bodyParser = require('body-parser');

  module.exports = function() {

    var app = express();

    app.use(bodyParser.urlencoded({extended: true}));
    app.use(bodyParser.json());

    consign()
     .include('controllers')
     .into(app);

  }

index.js

var app = require('./config/custom-express')();

app.listen(3000, function(){
  console.log("Servidor rodando!");
});

pagamento.js

module.exports = function(app){
  app.get('/pagamentos', function(req, res){
    console.log('Recebida requisicao de teste na porta 3000.')
    res.send('OK.');
  });

  app.post("/pagamentos/pagamento",function(req, res) {
        var pagamento = req.body;
        console.log(pagamento);
        res.send('ok');
    });

}