自定义指令抛出 - 错误：$ compile：ctreq缺少必需的控制器

Question

为什么当我使用ngModel作为require选项

时，require选项不会在我的指令中抛出错误

以下脚本正在运行

angular.module('myApp').directive('validNumber',[function(){
return{
    require:'ngModel',
    scope:{},
    link: function (scope ,element, attr, ngModel) {

    }
}}])      //here is my html <input ng-model="data" valid-number>

以下脚本无法正常工作原因？它会抛出编译错误缺少必需的控制器

angular.module('myApp').directive('validNumber',[function(){
return{
    require:'ngSandy',
    scope:{},
    link: function (scope ,element, attr, ngModel) {

    }
}}]) //here is my html <input ng-sandy="data" valid-number>