AngularJS checkbox ng-change issue with $event.target

I'm writing a simple AngularJS Controller that keeps track of the number of checkboxes checked. Trying to avoid $scope.$watch and instead use ng-change to increment/decrement the total count.

HTML:

<form ng-controller="MainCtrl">
  <table>
    <tr ng-repeat="item in data">
      <td>
      <input type="checkbox" 
             value="{{item.id}}"
             ng-model="item.selected"
             ng-change="updateTotal($event)"> &nbsp; {{item.name}}
       </td>
    </tr>
  </table>
  <p>
      Total checked: {{totalSelected}}
   </p>
</form>

Controller snippet

$scope.updateTotal = function($event) {

    var checkbox = $event.target;

    if (checkbox.checked) {
      $scope.totalSelected++;
    }
    else {
      $scope.totalSelected--;
    } 
}

I keep getting an error in the controller where I attempt to access $event.target:

TypeError: Cannot read property 'target' of undefined

I created a Plunk for recreating: http://ift.tt/1BRnKdu

If anyone has any ideas or suggestions I would be very grateful.

Thank you very much!