Validation not triggered when data binding a number input’s min / max attributes

I wrote directives to fill the gap, ng-min and ng-max:

http://jsfiddle.net/g/s5gKC/

var app = angular.module('app', []);

function isEmpty(value) {
  return angular.isUndefined(value) || value === '' || value === null || value !== value;
}

app.directive('ngMin', function() {
    return {
        restrict: 'A',
        require: 'ngModel',
        link: function(scope, elem, attr, ctrl) {
            scope.$watch(attr.ngMin, function(){
                ctrl.$setViewValue(ctrl.$viewValue);
            });
            var minValidator = function(value) {
              var min = scope.$eval(attr.ngMin) || 0;
              if (!isEmpty(value) && value < min) {
                ctrl.$setValidity('ngMin', false);
                return undefined;
              } else {
                ctrl.$setValidity('ngMin', true);
                return value;
              }
            };

            ctrl.$parsers.push(minValidator);
            ctrl.$formatters.push(minValidator);
        }
    };
});

app.directive('ngMax', function() {
    return {
        restrict: 'A',
        require: 'ngModel',
        link: function(scope, elem, attr, ctrl) {
            scope.$watch(attr.ngMax, function(){
                ctrl.$setViewValue(ctrl.$viewValue);
            });
            var maxValidator = function(value) {
              var max = scope.$eval(attr.ngMax) || Infinity;
              if (!isEmpty(value) && value > max) {
                ctrl.$setValidity('ngMax', false);
                return undefined;
              } else {
                ctrl.$setValidity('ngMax', true);
                return value;
              }
            };

            ctrl.$parsers.push(maxValidator);
            ctrl.$formatters.push(maxValidator);
        }
    };
});

angular.bootstrap(document.body, ['app']);