Why is my directive messing up my validations?

Question

I ripped this directive off from this SO question.

var directive = function() {
    return {
        require: 'ngModel',
        restrict: 'A',
        link: function(scope, element, attr, ctrl) {
            function inputValue(val) {
                if (val) {
                    var digits = val.replace(/[^0-9]/g, '');

                    if (digits !== val) {
                        ctrl.$setViewValue(digits);
                        ctrl.$render();
                    }
                    return parseInt(digits, 10);
                }
                return undefined;
            }
            ctrl.$parsers.push(inputValue);
        }
    };
};

module.exports = directive;

I want to limit my input to only accept numbers, but now my other validators, like min and max, are not working. My input is now always in an error state. What part of this directive is effecting the validation lifecycle in angular?