AngularJS - Close modal function not working

close and dismiss are methods of $modalInstance  object  returned by $modal.open:

$scope.open = function() {
    $scope.$modalInstance = $modal.open({
        scope: $scope,
        templateUrl: "modalContent.html",
        size: '',
    })
};

$scope.ok = function() {
    $scope.$modalInstance.close();
};

$scope.cancel = function() {
    $scope.$modalInstance.dismiss('cancel');
};