xmlHttp.responseXML.documentElement is null

[mesh2005]

I have a piece of javascript code that loads an XML, parses it and displays the result in a DIV element. This works perfectly in FF but in IE I'm getting the above error. The backend PHP has the following header:

header('Content-Type: text/xml');
header("Cache-Control: no-cache, must-revalidate");
echo "<?xml version='1.0'?>";

A sample of the XML:

<response>
<pages>1</pages>
<current>0</current>
<comment>
<date>2008-07-15</date>
<text>Test comment 1</text>
<poster>mesh2005</poster>
<link>http://www.yahoo.com</link>
</comment>
</response>

I added a Javascript alert statement to print the responseText and I can see the whole XML file is displayed correctly.

Please advice.

Thank you

Amir