Mocha diff not working with assert

index.js

/*global describe, it*/
'use strict';

var assert = require('assert');

describe('diff', function () {

  it('works', function () {
    assert.equal('foo', 'bar');
  });

});

output.txt

  diff
    1) works


  0 passing (9ms)
  1 failing

  1) diff works:
     AssertionError: "foo" == "bar"
      at Context.<anonymous> (/Users/max/projects/mocha-diff-bug/index.js:9:12)
      at callFn (/Users/max/projects/mocha-diff-bug/node_modules/mocha/lib/runnable.js:266:21)
      ...

package.json

{
  "name": "mocha-diff-bug",
  "version": "1.0.0",
  "scripts": {
    "test": "mocha index.js"
  },
  "devDependencies": {
    "mocha": "^2.2.1"
  }
}