Need help qureying MongoDB with nodeJS. Getting undefined is not a function on the following code

The following code is giving me undefined is not a function. My database has 22 numbers in it and I want to get the average. What am I doing wrong?

router.get('/mean', function(req, res) {
        var db = req.db;
        var collection = db.get('numbercollection');
        collection.aggregate([
        {$group : {_id : null, 
                   mean : {$avg : "$number"}
                  }   
        }], function(err, result) {
            console.log(result);
    });
    });

Just let me know if you need more information. Thanks.