node-horseman: `Unhandled reject Error: Failed to load url` when using SlimerJS

When using Horseman with SlimerJS the following error is returned:

>   horseman phantom created +637ms
  horseman phantom version 1.9.8 +147ms
  horseman page created +154ms
  horseman phantomjs onLoadFinished triggered +117ms [ 'success', 'about:blank', false ] NaN
  horseman .close(). +6ms
Unhandled rejection Error: Failed to load url
    at checkStatus (/Users/user/Projects/project/node_modules/node-horseman/lib/index.js:276:16)
    at tryCatcher (/Users/user/Projects/project/node_modules/bluebird/js/release/util.js:16:23)
    at Function.Promise.attempt.Promise.try (/Users/user/Projects/project/node_modules/bluebird/js/release/method.js:39:29)
    at Object.loadFinishedSetup [as onLoadFinished] (/Users/user/Projects/project/node_modules/node-horseman/lib/index.js:274:43)
    at /Users/user/Projects/project/node_modules/node-phantom-simple/node-phantom-simple.js:636:30
    at Array.forEach (native)
    at IncomingMessage.<anonymous> (/Users/user/Projects/project/node_modules/node-phantom-simple/node-phantom-simple.js:617:17)
    at emitNone (events.js:72:20)
    at IncomingMessage.emit (events.js:166:7)
    at endReadableNT (_stream_readable.js:905:12)
    at nextTickCallbackWith2Args (node.js:478:9)
    at process._tickDomainCallback (node.js:433:17)

The following test case will reproduce it:

const Horseman = require('node-horseman');

const config = {
  browser: {
    clientScripts: [],
    timeout: 5000,
    interval: 50,
    loadImages: true,
    switchToNewTab: false,
    // no support from SlimerJS cookiesFile: null,
    // no support from SlimerJS ignoreSSLErrors: false,
    sslProtocol: 'any',
    // no support from SlimerJS webSecurity: true,
    injectJquery: true,
    injectBluebird: false,
    bluebirdDebug: false,
    // no support from SlimerJS proxy: null,
    // no support from SlimerJS proxyType: null,
    // no support from SlimerJS proxyAuth: null,
    phantomPath: '/usr/local/Cellar/slimerjs/0.10.0/bin/slimerjs',
    debugPort: null,
    debugAutorun: true,
  },
  userAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_2) AppleWebKit/600.4.10 (KHTML, like Gecko) Version/8.0.4 Safari/600.4.10',
}

const horseman = new Horseman(config.browser);

horseman
  .userAgent(config.userAgent)
  .open('http://google.co.uk/')
  .screenshot('google.jpg')
  .close();