node mongo TypeError: boolean is not a function

db.open(... throws the following error

TypeError: boolean is not a function

The res function returns a function so not sure what I'm doing wrong here...

var db = new Db('test', new Server('localhost', 27017, {auto_reconnect: true}, {}));

function res(win) {
  return function(err,result){
    if( err ) {
      util.debug('mongo:err='+err);
      db.close();
    }
    else {
      util.debug('mongo:ok, about to call function');
      win(result);
    }
  }
}

exports.saveCreditCheck = function(req, res){
  util.debug('*** in saveCreditCheck ***');

  db.open(res(function(){
    util.debug('mongo:ok');
  }));

};

BTW, this works fine..

db.open(function(){
   util.debug('mongo:ok');
});