[Zope] getPreviousSibling

I have a list of folders containing thumbnail images. In each folder I make a link to the previous and next folder with "getPreviousSibling" and "getNextSibling". This works perfectly in all folders except for the first one. This is my code: <dtml-if getPreviousSibling> <dtml-with getPreviousSibling> <a href="<dtml-var absolute_url>"><< <dtml-var title_or_id></a> </dtml-with> </dtml-if> And Zope says: Error Type: SystemError Error Value: infinite recursion in document template My logic says <dtml-if getPreviousSibling> should return "none" for the first folder like <dtml-if getNextSibling> returns none for the last folder in the list. What could it be? roché