Unknown angular error when using sorting directive

Question

I have an angular directive that sorts columns in a table. prior to version 1.20 this worked but after upgrading to 1.20 I am getting the following error. Does anyone know what is wrong?

Error: [$parse:isecprv] http://errors.angularjs.org/undefined/$parse/isecprv?p0=_sort()

app.directive('sorted', function () {
    return {
        scope: true,
        transclude: true,
        template: '<a href="#/" ng-click="_sort()" ng-transclude=""></a>' +
            '<span class="sortArrow" ng-show="show_sort_icon(true)">&#x25BC;</span>' +
            '<span class="sortArrow" ng-show="show_sort_icon(false)">&#x25B2;</span>',
        controller: function ($scope, $element, $attrs) {

            $scope._sort = function () {
                $scope.model.sort($attrs.sorted);
            };

            $scope.show_sort_icon = function (is_desc) {
                return ($scope.model.sidx == $attrs.sorted) && ($scope.model.is_desc == is_desc);
            };
        }
    };
});

Usage:

<table>
    <thead>
        <tr>
            <th sorted="something">Something</th>
        </tr>
    </thead>
    <tbody>
        <tr ng-repeat="item in model.items">
            <td>{{item.something}}</td>
        </tr>
    </tbody>
</table>

Answer 1

Answer found by @Heikki

Fields with names that begin or end with an underscore are considered private fields. Angular expressions are not allowed to reference such fields on the scope chain. This only applies to Angular expressions (e.g. interpolation and calls to $parse with a string expression argument) – Javascript itself has no such notion.

To resolve this error, use an alternate non-private field if available or make the field public (by removing any leading and trailing underscore characters from its name.)

Example expression that would result in this error:

<div>{{user._private_field}}</div>