Can't change desktop to XFCE

Hello,

I have Knoppix 7.04 DVD and want to boot into XFCE.

I'm able to boot into KDE by typing
knoppix desktop=kde

But when I try either
knoppix desktop=xfce, or
knoppix desktop=xfce4
it defaults to LXDE.

Can you tell me what I'm doing wrong?

Thanks