ui-bootstrap tooltip doesn't work when used with ng-repeat

I would like to use the ui-bootstrap tooltip directive to assign tooltips to an array of buttons which are rendered by an ng-repeat directive, like this:

<button ng-repeat="label in labels" tooltip="{{label}}">{{label}}</button>

but I'm facing a strange issue: the ng-repeat loop variable gets a blank value, except that in the tooltip itself, so for example in the above example the label value would be blank in the button. A snippet showing the issue could be seen at this link. Everything is ok removing the tooltip attribute. Is there anyone who has been using ui-bootstrap and knows about such issues?