I keep getting "Error: Arguments must be aggregate pipeline operators" but I checked out the statement using mongo shell

try{
    Staff.aggregate(
            {$group:{_id:{name:"$name",age:'$age'},total:{$sum:1}}}, 
            {$match:{total:{$gt:1}}}
    , modelHelper.mergeStaffCallBack);
}catch (err) {
   console.log(err);
}

This is the equivalent query in mongoshell.

db.staffs.aggregate([
    {
        $group:{
            _id:{name:"$name",age:'$age'},
            total:{$sum:1}
        }
    },
    {
        $match:{
             total:{ $gt: 1 }
        }
    }
]);

It executed successfully. I cannot spot the error in my nodejs code above. The catch caught the error

"Error: Arguments must be aggregate pipeline operators".

Can someone enlighten me?