ASync 怎么实现异步嵌套啊？

sites.forEach(function (site) { async.map(site.channel,function(channel,callback){ callback(null,channel) },function(err,results){ async.map(results,function(result,callback){

            getrequest(result.link,function(list){
                callback(null,list);
            });

        },function(err,lists){
              console.log(lists);
        });
    });
	

报这个错：if (called) throw new Error("Callback was already called.");