It seems if you do not have a title value for hyperlink with Lightbox , Internet Explorer displays a null value for the default description Lightbox has released an update and the changelog fixes this error

• “FIXED – Caption displays as “null” when viewing an uncaptioned image after viewing a captioned image.”

However if you are using a third party tool, where you cannot modify the main lightbox files , you can resolve this issue by setting the title flag yourself in the hyperlink to nothing e.g.

<a href=’picture.gif’ rel=’lightbox’ title=””><img src=”thumbpic.gif” border=”0″></a><br><a class=’linkHiRes’ href=’pricingpage.gif’></a>