Angularjs报错：TypeError: Cannot read property 'slice' of undefined

报错概要：

TypeError: Cannot read property 'slice' of undefined

报错内容：

angular.js:12450 TypeError: Cannot read property 'slice' of undefined
    at controller.js:1383
    at fn (eval at compile (angular.js:13275), :4:217)
    at regularInterceptedExpression (angular.js:14335)
    at Object.expressionInputWatch [as get] (angular.js:14237)
    at Scope.$digest (angular.js:15751)
    at Scope.$apply (angular.js:16030)
    at done (angular.js:10545)
    at completeRequest (angular.js:10717)
    at XMLHttpRequest.requestLoaded (angular.js:10658)

出错源码：

app.filter('split_last_letter',function() { //可以注入依赖
    return function(text) {

            return text.slice(0,-2)
    };
});

修正源码：

app.filter('split_last_letter',function() { //可以注入依赖
    return function(text) {
            if (!text) { return; }  
            return text.slice(0,-2)
    };
});

错误分析：

当angular 第一次执行 $digsest 循环的时候，它会执行自定义的过滤器，而此时是没有值的所有会报错