Cannot read property 'checked' of null in TestCase

Question

In my JS I have below mentioned code in Initialize function.

initialize: function () {

        if (!$("#VisitorIsSelected").is(':checked'))
        {
            $("#VisitorIsSelected").attr('checked', true);
        }
}

But when I run below mentioned Test Case via Qunit.js and blanket.js it shows me error :Cannot read property 'checked' of null - {}"

test("initialize test", 3, function () {
    $('<input>').appendTo("body");
    $('<select>').appendTo("body");
    $('<input>', { type: "checkbox", checked: true, id: "VisitorIsSelected"}).appendTo("body");
    var result = initialize();
    equal(undefined, result, "passed");

    $('input, select').trigger("blur");    
    ok(true, "blur event called");
    $('input, select').trigger("change");
    ok(true, "change event called");
    $("input").remove();
    $("select").remove();
});

What should be the exact test case?