function onBridgeReady(){
        WeixinJSBridge.invoke(
                'getBrandWCPayRequest',{
                   "appId": "<%= @r['appid']%>",
                     "timeStamp": "<%= Time.now.to_i.to_s %>",
                     "nonceStr": "<%= @r['nonce_str']%>",
                     "package": "prepay_id=<%= @r['prepay_id']%>",
                     "signType": "MD5",
                     "paySign": "<%= @r['sign'] %>"
                 },
                 function(res){
alert(res.err_msg);
                    if(res.err_msg == "get_brand_wcpay_request:ok"){
                            alert('ok');
                        }else if(res.err_msg == "get_brand_wcpay_request:cancel") {
                        }else if(res.err_msg == "get_brand_wcpay_request:fail"){
                            alert('fail');
                        }else{
                            } 
                 }
            );   
     }
     if (typeof WeixinJSBride == "undefined"){
         if (document.addEventListener){
             document.addEventListener('WeixinJSBridgeReady',onBridgeReady,false); 
            } else if (document.attachEvent){
             document.attachEvent('WeixinJSBridgeReady',onBridgeReady);
             document.attachEvent('onWeixinJSBridegReady',onBridgeReady);
            }
        }else{
          onBridgeReady();  
        }

请问 fail 是什么原因？

get_brand_wcpay_request:fail 怎么解