If input field is empty, disable submit button

I’m trying to implement a very simple form with one field and a submit button. If the field is empty, then submit have to be disabled.

In my page, I have (in the text mode):

jQuery('#myform > input').on('input', function () {
        var empty = false;
        $('form > input').each(function () {
            if ($(this).val() == '') {
                empty = true;
            }
        });

        if (empty) {
            $('#register').attr('disabled', 'disabled');
        } else {
            $('#register').removeAttr('disabled');
        }
});

<form id="myform">
    <input type="text" id="user_input" name="username" placeholder="enter ..." />
    <input type="submit" id="register" value="Register" disabled="disabled" />
</form>

But my submit button is always disabled.
However, my jsfiddle works fine !
http://jsfiddle.net/x25pv/24/