Track by isn't working

[climboid]

Specified track by $index on the slide directive but I still get the same error

<div class="flexslider homePageMainSlider">
  <flex-slider slide="s in topHomeCarousel track by $index" class="slides" direction-nav="false" animation="slide" control-nav="false">
  <li>
      <img ng-src="{{s}}">
  </li>
  </flex-slider>
</div>

What does work is if I use a property of the array of objects I'm looping through for instance

<div class="flexslider homePageMainSlider">
  <flex-slider slide="s in topHomeCarousel track by s.id" class="slides" direction-nav="false" animation="slide" control-nav="false">
  <li>
      <img ng-src="{{s}}">
  </li>
  </flex-slider>
</div>

[apptv]

Confirmed.
Same for me.