Document not found when read_secondary is true

gistfile1.js

var mongoose = require('mongoose');

var mongoConfig = {
    servers : [
        'localhost:27017/test-replset'
      , 'localhost:27018'
      , 'localhost:27019'
    ]
  , options : {
        server : {
            auto_reconnect: true
        }
      , replset : {
            rs_name         : 'rs0'
          , read_secondary  : true
        }
      , db : {
            safe  : { w: 3, wtimeout: 5000 }
        }
    }
};

var File = new mongoose.Schema({
      servername      : { type: String, index: { unique: true } }
    , originalname    : String
});

var FileModel = mongoose.model('File', File);

function connect(callback) {
    var connStr = 'mongodb://'+mongoConfig.servers.join(',');
    mongoose.connect(connStr, mongoConfig.options, function(err) {
        if(err) {
            console.log(err);
            return;
        }
        console.log(err || "Connected to MongoDB");
        callback();
    });
}

connect(function() {
    FileModel.remove({}, function(err) {
        if(err) {
            error(err || new Error('Error emptying collection'));
            return;
        }
        function request(i, callback) {
            var file = new FileModel({
                servername    : "Hello "+i
              , originalname  : "Word "+i
            });
            file.save(function(serr) {
                if(serr) {
                    console.log('error while saving', serr);
                    return;
                }
                console.log("looking for "+file.servername);
                FileModel.findOne({servername: file.servername}, function(err, doc) {
                    if(err || !doc) {
                        callback(err || new Error('File not found'));
                        return;
                    }
                    callback(null, doc);
                });
            });
        }
        function go(i) {
            request(i, function(err, doc) {
                if(err) console.log("ERROR! ",err.message);
                else console.log("success!");
                if(i-->0) go(i);
            });
        }
        go(10);
    });
});