[Zope] newbie-prob: dtml-in ... sort=myVar

hi, a page is called via links like 1.) myPage.html?sortkey=Date 2.) myPage.html?sortkey=Name the value of sortkey is display properly with <dtml-var sortkey> i like to use the value of sortkey within the <dtml-in> -tag, like <dtml-in expr="objectItems(['DTML Document'])" sort=_['sortkey']> but that is not working... the tags <dtml-in expr="objectItems(['DTML Document'])" sort=Date> <dtml-in expr="objectItems(['DTML Document'])" sort=Name> are doing fine, but that's not a solution for my prop. please help thanks in advance cheers, Markus