this.skip causes: Cannot read property 'toString' of undefined

[ryan-shaw]

Error line in mocha.js

    var el;
    if (test.state === 'passed') {
      var url = self.testURL(test);
      el = fragment('<li class="test pass %e"><h2>%e<span class="duration">%ems</span> <a href="%s" class="replay">‣</a></h2></li>', test.speed, test.title, test.duration, url);
    } else if (test.pending) {
      el = fragment('<li class="test pass pending"><h2>%e</h2></li>', test.title);
    } else {
      el = fragment('<li class="test fail"><h2>%e <a href="%e" class="replay">‣</a></h2></li>', test.title, self.testURL(test));
      var stackString; // Note: Includes leading newline
      var message = test.err.toString(); // mocha.js:2427

My test cases are

describe('mod', function(){
  describe('mod1', function(){
    before(function(){
       if(!modLoaded) this.skip(); // causes the error
    });

    it('should do stuff', function(){
       // stuff
    });
  });
});

Not sure why this.skip() is causing an error...