这里发生了什么？

这是我的指令：

app.directive('submitRequired', function (objSvc) {
    return {
        require: 'ngModel',
        link: function (scope, elm, attrs, ctrl) {

          // do something
        }
    };
});

这是使用中的指令的示例：

<input submit-required="true"></input>

这是实际的错误文本：

Error: [$compile:ctreq] Controller 'ngModel', required by directive 'submitRequired', can't be found!
http://errors.angularjs.org/1.2.2/$compile/ctreq?p0=ngModel&p1=submitRequired
    at http://www.domain.ca/Scripts/angular/angular.js:78:12
    at getControllers (http://www.domain.ca/Scripts/angular/angular.js:5972:19)
    at nodeLinkFn (http://www.domain.ca/Scripts/angular/angular.js:6139:35)
    at compositeLinkFn (http://www.domain.ca/Scripts/angular/angular.js:5550:15)
    at nodeLinkFn (http://www.domain.ca/Scripts/angular/angular.js:6132:24)
    at compositeLinkFn (http://www.domain.ca/Scripts/angular/angular.js:5550:15)
    at publicLinkFn (http://www.domain.ca/Scripts/angular/angular.js:5458:30)
    at http://www.domain.ca/Scripts/angular/angular.js:1299:27
    at Scope.$get.Scope.$eval (http://www.domain.ca/Scripts/angular/angular.js:11634:28)
    at Scope.$get.Scope.$apply (http://www.domain.ca/Scripts/angular/angular.js:11734:23) <input submit-required="true"> angular.js:9159
(anonymous function) angular.js:9159
$get angular.js:6751
nodeLinkFn angular.js:6141
compositeLinkFn angular.js:5550
nodeLinkFn angular.js:6132
compositeLinkFn angular.js:5550
publicLinkFn angular.js:5458
(anonymous function) angular.js:1299
$get.Scope.$eval angular.js:11634
$get.Scope.$apply angular.js:11734
(anonymous function) angular.js:1297
invoke angular.js:3633
doBootstrap angular.js:1295
bootstrap angular.js:1309
angularInit angular.js:1258
(anonymous function) angular.js:20210
trigger angular.js:2315
(anonymous function) angular.js:2579
forEach angular.js:300
eventHandler angular.js:2578ar.js:7874