How to initially disable custom button

I've added a new custom button using the code below. I want the button to be disabled when the editor is initially loaded. How to achieve this ? I tried to set the state to CKEDITOR.TRISTATE_DISABLED (see code below) but this doesn't work.

CKEDITOR.plugins.add('myButton',
{
     init: function(editor)
     {
          var pluginName = 'myButton';
          editor.addCommand('helloworld', {exec:helloWorld});
          editor.getCommand('helloworld').setState(CKEDITOR.TRISTATE_DISABLED);
          editor.ui.addButton('myButton',
               {
                    label: 'New button',
                    command: 'helloworld',
                    icon: '/plugins/myButton/button.gif'
               });
    }
});

function helloWorld(e)
{
     alert('Hello world');
}