cappedMax não funciona no logger winston-mongodb no Node.js no Ubuntu

var winston = require('winston');
require('winston-mongodb').MongoDB;


winston.loggers.add('userLog',{
    transports : [
        new(winston.transports.MongoDB)({
            db : 'mongodb://username:password.mongolab.com:5555/log_db',
            collection : 'userLog',
            capped : true
        }),
    ]
});
winston.loggers.add('profileLog',{
    transports : [
        new(winston.transports.MongoDB)({
            db : 'mongodb://username:password.mongolab.com:5555/log_db',                collection : 'profileLog',
            capped : true
        }),
    ]
});