[Zope] dtml-if Newbie Question

Hi,

From the 'Zope Book' the statement

<dtml-if variable> means that 'if the..variable does not exist, then this condition is false'. I found that this is not true. When I tried using dtml-if to a non-existent variable, I got a 'key error'. So how do I check if the variable does not exist ? Tks. AL