AngularJS Form custom validation. Cannot read property '$validators' of undefined

I want to create a custom form validation, using AngularJS. That form should have input and select elements. The form should be valid, when either imputs are empty or both filled with some values. Here is the view:

<form name="recipientsForm" novalidate>
    <md-input-container>
        <label>Name</label>
        <input name="name" type="text" ng-model="relationship.name" value="" empty-or-both-filled="relationship.relationshipType">
        <div ng-messages="recipientsForm.name.$error">
            <div ng-message="emptyOrBothFilled">Enter name.</div>
        </div>
    </md-input-container>
    <md-input-container>
        <md-select name="type" placeholder="Select your relation... " ng-model="relationship.relationshipType" empty-or-both-filled="relationship.name">
            <md-option ng-repeat="type in relationshipTypes" value="{{type.relationshipType}}">
                {{type.name}}
            </md-option>
        </md-select>
        <div ng-messages="recipientsForm.type.$error">
            <div ng-message="emptyOrBothFilled">Pick relationship.</div>
        </div>
    </md-input-container>
</form>

And here is the directive:

(function () {
    'use strict';

    angular
        .module('app')
        .directive('emptyOrBothFilled', [emptyOrBothFilled]);

    function emptyOrBothFilled() {
        return {
            restrict: 'A',
            required: 'ngModel',
            scope: {
                targetNgModel: '=emptyOrBothFilled'
            },
            link: function($scope, element, attrs, ngModel) {
                ngModel.$validators.emptyOrBothFilled = function(val) {
                    var isValueEmpty = !val;
                    var isTargetEmpty = !$scope.targetNgModel;

                    return (isTargetEmpty && isValueEmpty) || (!isTargetEmpty && !isValueEmpty);
                }

                $scope.$watch('targetNgModel', function() {
                    ngModel.$validate();
                })
            }
        }
    }
})();

Prompt, please, why do I get this error:

TypeError: Cannot read property '$validators' of undefined
    at link (http://localhost:3000/app/shared/directives/EmptyOrBothFilled.js:17:24)