[Zope] empty multiple lists

Hi everybody, although this may have been discussed several times: I added a multiple property to a class, but when I call the instance's property sheet from the management interface, I get an Type Error, whenever I did not chose anything from the selection box. That is of course ok, because there is no property defined within the instance if I did not select anything. (getProperty(id) fails on None) How do I create an "empty selection"? Thanks for your help BTW: <dtml-call "REQUEST.set('x','[]')"> did not work.