fb is undefined

I have a problem with Internet Explorer. But before proceeding my question let me give you the details.

1. I have wordpress installed on my website.

2. I installed following plugin: Facebook Comments for WordPress (http://we8u.com/facebook-comments/)

This plugin simply enable users to post comment with their Facebook accounts without any registration on my site.

So the plugin works perfect on Chrome, Firofox, Safari and everywhere else except Internet Explorer. Actually it works on Internet Explorer as well but it gives this annoying error message which i would like to get rid of:

Webpage error details Message: 'FB' is undefined Line: 253 Char: 2 Code: 0

So i searched the plugin's forum for help and found this:

"If the comments still aren't appearing in IE, open up your theme's header.php file and add the following attributes to the tag: xmlns:fb="http://www.facebook.com/2008/fbml" xmlns:og="http://opengraphprotocol.org/schema/""

I added that attribute but still same error message. This is only happening on IE.

Does anyone have any suggestion?

Upgrading ie to the latest version solved the problem.