nth-of-type broken in IE6

```
<div id="xid">
    <div id="one">1</div>
    <div id="two">2</div>
    <span id="aspan"></span>
    <div id="three">3</div>
    <img src="" />
    <div id="four">4</div>
</div>

$('div#xid > div:nth-of-type(3)');
```

this should match the div id="three" but it doesnt. it just breaks on the span and returns nothing.
