[Zope] getPreviousSibling

[RC Compaan]

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