[Zope] Test of existance of attributes

The code below do not works as I through: when the attribute expiredate is not defined, it raise an error instead of display the sequence-item. How can I detect the existence of the attribute before comparision? Rgs, Kent Sin PS the code : <dtml-in "objectValues(['DTML Document'])" sort="id" skip_unauthorized> <dtml-if "expiredate and (expiredate - ZopeTime() ) >= 0.0 and not deleted"> <h2><dtml-var title_or_id></h2> <dtml-if "AUTHENTICATED_USER.has_permission( 'Change DTML Documents', _['sequence-item'])"> <a href="manage_edit">Edit</a> </dtml-if> <dtml-if "(ZopeTime() - _['bobobase_modification_time']) > Newdate"> <em>New</em> </dtml-if> <dtml-var sequence-item fmt="structured-text"> </dtml-if> </dtml-in>