Mocked function not found in AngularUI modal controller unit test

I'm using Angular-Bootstrap modals and have a basic controller like so:

.controller('DashboardHelpController', ['$scope', '$uibModal', function ($scope, $uibModal) {

    var dhc = this;

    dhc.open = function (size, resource) {
        var modalInstance = $uibModal.open({
            templateUrl: 'resourcePlayModal.html',
            controller: 'ModalInstanceCtrl as mic',
            size: size,
            resolve: {
                resource: function () {
                    return resource;
                }
            }
        });
    };
}])

It calls a standard modal instance controller:

.controller('ModalInstanceCtrl', ['$uibModalInstance', 'resource', function ($uibModalInstance, resource) {
    this.resource = resource;

    this.cancel = function () {
        $uibModalInstance.dismiss();
    };
}])

And here's my unit test, modeled after another SO post:

describe('Modal controller', function () {
    var modalCtrl, scope, modalInstance;

    beforeEach(module('MyApp'));

    // initialize the controller and a mock scope
    beforeEach(inject(function ($controller, $rootScope) {
        scope = $rootScope.$new();

        modalInstance = {
            // create a mock object using spies
            close: jasmine.createSpy('modalInstance.close'),
            dismiss: jasmine.createSpy('modalInstance.dismiss'),
            result: {
                then: jasmine.createSpy('modalInstance.result.then')
            }
        };

        modalCtrl = $controller('DashboardHelpController', {
            $scope: scope,
            $uibModalInstance: modalInstance
        });
    }));

    it('should instantiate the mock controller', function () {
        expect(modalCtrl).not.toBeUndefined();
    });

    it('should have called the modal dismiss function', function () {
        scope.cancel;
        expect(modalInstance.dismiss).toHaveBeenCalledWith();
    });
});

The problem is that the cancel function isn't found on the scope:

Expected spy modalInstance.dismiss to have been called with [ 'cancel' ] but it was never called.

Things are complicated a bit my my use of the Controller as syntax, but this should work. Thanks for any help.