[TypeError: Cannot call method 'call' of undefined]

promise.js

var Browser = require("zombie");
var expect = require("expect.js");

browser = new Browser();
browser.visit("http://somedomain.com").
then(function () {
        console.log("yo");
        return fill("username", "monkeyeater").fill("password", "ispassed");
}).
then(function () {
        return pressButton("input#loginbtn")
}).
then(function () {
        // Form submitted, new page loaded.
        expect(browser.success).to.be.ok();
        expect(browser.text("title")).to.be("UR Courses");
}).fail(function(err){
        console.log(err);
});

//[TypeError: Cannot call method 'call' of undefined]