When my XAJAX call, via PHP, returns NULL - it gets displayed on IE (but it should display nothing), how to correct this?

Please review the code fragments below:

First, the Javascript call:

function getNewSelect(property_id){
  xajax_getSelectDropDown(unique_id);
  $(".chzn-select").chosen();
}

Next, the PHP XAJAX function:

function getSelectDropDown($unique_id){
    //CODE ABOVE THIS POINT HAS BEEN REMOVED, IT IS IRRELEVENT FOR THIS EXAMPLE

        //Code for if no matches were found:
        $html2 = NULL;

        $objResponse = new xajaxResponse();
        if(mysql_num_rows($result) > 0){
            $objResponse->assign('subDiv', 'innerHTML', $html);
        }else{
            $objResponse->assign('subDiv', 'innerHTML', $html2);
        }

        $objResponse->script("$('.chzn-select-ajax').chosen();");

    return $objResponse;
}

Finally here's the HTML code:

<div id="subDiv"></div>

Now, when the above PHP function goes into the else and assigns $html2 - I expect nothing to be displayed to the user. On most browsers, that's fine and it works perfectly. Unfortunately on internet explorer (IE 9 I can confirm, I haven't been able to test other versions), it actually displays the following where the DIV call:

null

If a result is found and it does not process the else, then it works correctly on all browsers and shows a new drop down as it's supposed to.

What can I do to fix this problem? Is this a known issue with IE? Thanks.

I want to add so there's no confusion, the CHZN call is just to add a style to the dropdown, as found here: http://harvesthq.github.com/chosen/

Better use this:

    if(mysql_num_rows($result) > 0){
        $objResponse->assign('subDiv', 'innerHTML', $html);
    }else{
        $objResponse->assign('subDiv', 'innerHTML', '');
    }

IE is showing null because $html2 = NULL......better try it with $html2 = " " or something like that ;)

Saludos.