/**
 * 微信支付
 * @link https://pay.weixin.qq.com/wiki/doc/api/jsapi.php?chapter=7_7&index=6
 */
var wxPay = {
    to: function() {
        if (typeof WeixinJSBridge == "undefined"){
            if( document.addEventListener ){
                document.addEventListener('WeixinJSBridgeReady', wxPay._onBridgeReady, false);
            }else if (document.attachEvent){
                document.attachEvent('WeixinJSBridgeReady', wxPay._onBridgeReady);
                document.attachEvent('onWeixinJSBridgeReady', wxPay._onBridgeReady);
            }
        }else{
            wxPay._onBridgeReady();
        }
    },
    ok: function(ctx) {
        console.log(ctx);
    },
    cancel: function(ctx) {
        console.log(ctx);
    },
    fail: function(ctx) {
        console.log(ctx);
    },
    other: function(ctx) {
        console.log(ctx);
    },
    brandWCPayRequest:{
        appId:'',//公众号名称，由商户传入
        timeStamp:'',//时间戳，自1970年以来的秒数
        nonceStr:'',//随机串
        package:'', //订单详情扩展字符串
        signType:'',//微信签名方式
        paySign:'',//微信签名
    },
    _onBridgeReady: function () {
        try {
            WeixinJSBridge.invoke('getBrandWCPayRequest', wxPay.brandWCPayRequest, function(res) {
                switch (res.err_msg) {
                    case 'get_brand_wcpay_request:ok': //支付成功
                        wxPay.ok({
                            result: res,
                            param: wxPay.brandWCPayRequest
                        });
                        break;
                    case 'get_brand_wcpay_request:cancel': //支付过程中用户取消
                        wxPay.cancel({
                            result: res,
                            param: wxPay.brandWCPayRequest
                        });
                        break;
                    case 'get_brand_wcpay_request:fail': //    支付失败
                        wxPay.fail({
                            result: res,
                            param: wxPay.brandWCPayRequest
                        });
                        break;
                    default:
                        wxPay.other({
                            result: res,
                            param: wxPay.brandWCPayRequest
                        });
                }
            });
        }catch (e) {
            wxPay.other({
                exception:e.toString(),
                param: wxPay.brandWCPayRequest
            });
        }
    }
};

以上是我的支付js，偶尔会发生异常：system:function_not_implement,请问是为什么呢