how do correctly the routing?

how do correctly the routing

Client:

controller

$scope.exportData = function () {
    $http.get('/api/customers/excel/');
}; 

Server:

controller

var Customer = require('./customer.model');
var excelReport = require('excel-report');
var express = require('express');
var _ = require('lodash');
var fs = require('fs');

// Get list of customers
exports.index = function(req, res) {
  Customer.find(function (err, customers) {
    if(err) { return handleError(res, err); }
    return res.json(200, customers);
  });
};

//calling with index
exports.excel = function(req, res) {
  console.log('test export');
  console.log(__dirname + 'client/template.xlsx');
  var template_file ='template.xlsx';
  res.download(template_file);
  });
};

index (index routes)

var express = require('express');
var controller = require('./customer.controller');

var router = express.Router();

router.get('/', controller.index);
router.get('/excel', controller.excel);

module.exports = router;

routes.

module.exports = function(app) {

  // Insert routes below
  app.use('/api/customers', require('./api/customer'));
  app.use('/api/things', require('./api/thing'));


  // All other routes should redirect to the index.html
  app.route('/*')
    .get(function(req, res) {
      res.sendfile(app.get('appPath') + '/index.html');
    });
}

i use generator-angular-fullstack for creating project to test

but I have the following error

GET http://localhost:9000/api/customers/excel/ 500

helpme please