Data from directive not displaying within ng-repeat

I have broken this problem down into it's simplest form. Basically I have a directive that, for the demo, doesn't yet really do anything. I have a div with the directive as an attribute. The values within the div, which come from an object array, are not displayed. If I remove the directive from the div, they are displayed OK. I am clearly missing something really obvious here as I have done this before without any problems.

Here's the Plunk: http://plnkr.co/edit/ZUXD4qW5hXvB7y9RG6sB?p=preview

Script:

app.controller('MainCtrl', function($scope) {

  $scope.tooltips = [{"id":1,"warn":true},{"id":2,"warn":false},{"id":3,"warn":true},{"id":4,"warn":true}];

});

app.directive("cmTooltip", function () {
    return {
        scope: {
            cmTooltip: "="
        }
    };
});

HTML

<div ng-repeat="tip in tooltips" class="titlecell" cm-tooltip="true">
    A div element: {{ tip.id }}
</div>
<br><br>

Just to prove it works without the directive:
<div ng-repeat="tip in tooltips" class="titlecell">
    A div element: {{ tip.id }}
</div>