MercadoShop JS error in final step of basic checkout

I am using the sample that is on the MP page, but when I try to finish the final step of the checkout (in sandbox), I get a javascript error.

var MP = require ("mercadopago"),

exports.run = function (req, res) {
    var mp = new MP ("CLIENT_ID", "CLIENT_SECRET"); // Nota: tengo el CLIENT_ID y el CLIENT_SECRET y funcionan correctamente.

    var preference = {
        "items": [
            {
                "title": "Multicolor kite",
                "quantity": 1,
                "currency_id": "UYU", // Available currencies at: https://api.mercadopago.com/currencies
                "unit_price": 1000.0
            }
        ]
    };

    mp.createPreference (preference, function (err, data){
        if (err) {
            res.send (err);
        } else {
            res.render ("button", {"preference": data});
        }
    });
};

/* view button.jade
!!! 5
html
    head
        Pay

    body
        (href=preference.response.sandbox_init_point) Pay
*/

When I give it to him to pay, I skip the error of JS as it is seen in the image:

Uncaught TypeError: Cannot read property 'replace' of undefined
    at getNumberValue (formStepBundle.js.825f915608de.gz:formatted:435)
    at callJsonp (formStepBundle.js.825f915608de.gz:formatted:1007)
    at d.<anonymous> (formStepBundle.js.825f915608de.gz:formatted:852)
    at d.b.emit (chico.min.js:1)
    at d.validate (chico.min.js:1)
    at HTMLFormElement.<anonymous> (chico.min.js:1)
    at HTMLFormElement.dispatch (jquery-1.10.2.min.js:5)
    at HTMLFormElement.v.handle (jquery-1.10.2.min.js:5)

Any help is appreciated.