Can't connect to localhost: Using CasperJS for ExpressJS application test suite

I seem to be having a very basic issue with using CasperJS to create a functional test suite for an ExpressJS based web application. I can't seem to connect to t

I created a barebones Express app to test using the Express executable, e.g. express myapp. I start the app with node app.js.

The below is the test script, what I want to do is simply connect to my Express page and check the title, which should be "Express".

var casper = require("casper").create({
        logLevel: "debug"
});

casper.start("localhost:3000/")

casper.then(function() {
        this.test.assertTitle("Express", "Express homepage title is the one expected");
});
casper.run(function() {
        this.exit();
});

When I am trying to connect to localhost, as above, this is the error message I receive. It seems that casper can't find a title.

FAIL Express homepage title is the one expected
#    type: assertTitle
#    subject: ""
#    expected: "Express"

If I change to trying to connect to my server's external domain, e.g. casper.start("http://mydomain.com:3000/"), then the test passes.

PASS Express homepage title is the one expected

What is going on here?