disabled property and disable method doesn't work

1. The version(s) of jQuery affected: jquery-1.4.2

2. The browser (or browsers) that you are able to reproduce the bug in, including version numbers: IE 9.0.81112, IE 7.0.5730, Firefox 3.6.8

3. The operating system (or operating systems) you experienced the bug on: Windows XP, Windows 7

4. Step-by-step instructions on how to reproduce the issue, including any required system configuration changes:

<div class="layout-slider">

<input id="SliderSingle2" type="slider" name="price" value="20" />

</div>

<script type="text/javascript" charset="utf-8">

jQuery("#SliderSingle2").slider({ disabled:true, from: 5, to: 55, step: 2.5, round: 1, dimension: ' €', skin: "round" });

jQuery("#SliderSingle2").slider('disable'); // <<<< DOESN'T WORK

</script>

5. A description of what you expect to happen, and what actually happens