simple dialog - panel and checkboxes

I have very simple dialog - just one panel and few checkboxes. When I set panel as "undefined" everything works right, but when i set a size of the panel checkboxes disappear.

Works

var win = new Window ("dialog", "test");
var panelOpcje = win.add("panel", undefined, "Options");
var check1 = panelOpcje.add("checkbox", undefined, "check1");
var check2 = panelOpcje.add("checkbox", undefined, "check1");
win.show();

This doesn't work

var win = new Window ("dialog", "test");
var panelOpcje = win.add("panel", [0,0,400,300], "Options");
var check1 = panelOpcje.add("checkbox", undefined, "check1");
var check2 = panelOpcje.add("checkbox", undefined, "check1");
win.show();

What am I doing wrong, where are my checkboxes?