How to test asychronous errors?

I am using mocha and should to test a simple spike node server.

exports.start = function(portNumber, htmlFilePath) {
    if(!portNumber) throw new Error('No port number has been passed.');

    server = http.createServer();

    server.on('request', function(request, response) {
        if(request.url === '/') {
            if(!htmlFilePath) throw new Error('No html file path has been passed.');

            fs.readFile(htmlFilePath, function (err, data) {
                response.end(data);
            });
        } else {
            response.statusCode = 404;
            response.end();
        }
    });

    server.listen(portNumber);
};

How would I go about testing that when the request url is "/" and that no htmlFilePath was passed it should throw an error?

it('should throw an error if no html file path was passed and asked for "/"', function(done) {
    server.start(PORT_NUMBER);
    (function() {
        http.get('http://localhost:' + PORT_NUMBER, function() {
            done();
        });
    }).should.throw();
});

This test does not pass, as the error is thrown asynchronously:

AssertionError: expected [Function] to throw exception

Any ideas?