Simplest fez-file, doesn't work :(

fez.js

'use strict';

var fez = require('fez');
var concat = require('fez-concat');

exports.build = function (stage) {
  stage(function (rule) {
    rule('app/assets/javascripts/*.js', 'dist.js', concat());
  });
};

exports.default = exports.build;

fez(module);

Gives error

[ { file: 'app/assets/javascripts/application.js',
    outputs: [ [Object] ],
    inputs: [] },
  { fn: [Function: concat],
    inputs: [ [Object], [Object], [Object], [Object], [Object] ],
    inComplete: 0,
    always: undefined,
    _id: 0,
    output: { file: 'dist.js', outputs: [], inputs: [Object] } },
  { file: 'dist.js', outputs: [], inputs: [ [Object] ] },
  { file: 'app/assets/javascripts/application_test.js',
    outputs: [ [Object] ],
    inputs: [] },
  { file: 'app/assets/javascripts/xxx.js',
    outputs: [ [Object] ],
    inputs: [] },
  { file: 'app/assets/javascripts/xxx_mocks.js',
    outputs: [ [Object] ],
    inputs: [] },
  { file: 'app/assets/javascripts/filters.js',
    outputs: [ [Object] ],
    inputs: [] } ]
app/assets/javascripts/application.js app/assets/javascripts/application_test.js app/assets/javascripts/xxx.js app/assets/javascripts/xxx_mocks.js app/assets/javascripts/filters.js -> dist.js
Rejected: TypeError: Expecting a function in instanceof check, but got //= require jquery
//= require lodash
//
// SNIP concatenated contents of my files here
//
});



    at processOutput (/Users/jan/development/contentful/user_interface/node_modules/fez/src/main.js:338:28)
    at /Users/jan/development/contentful/user_interface/node_modules/fez/src/main.js:336:14
    at tryCatch1 (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/util.js:85:19)
    at Promise$_callHandler [as _callHandler] (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/promise.js:677:13)
    at Promise$_settlePromiseFromHandler [as _settlePromiseFromHandler] (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/promise.js:693:18)
    at Promise$_settlePromiseAt [as _settlePromiseAt] (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/promise.js:875:14)
    at Promise$_fulfillPromises [as _fulfillPromises] (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/promise.js:976:14)
    at Async$_consumeFunctionBuffer [as _consumeFunctionBuffer] (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/async.js:63:12)
    at Async$consumeFunctionBuffer (/Users/jan/development/contentful/user_interface/node_modules/fez-concat/node_modules/bluebird/js/main/async.js:36:14)
    at process.startup.processNextTick.process._tickCallback (node.js:245:9)
An operation has failed. Aborting.