ejs.filters is not working

I'm trying to understand ejs filters, but I can't get it working:

Server.js

var ejs  = require('ejs');

ejs.filters.test = function() {

//empty for test

};

Routes.js

app.get('/index', function(req, res) {

    res.render('index.ejs', {
        data : someData
    });

});

Index.ejs:

<% test %>

Error:

...test is not defined 
at buf.push.__stack.lineno (eval at <anonymous> ....