[Zope] error_type not defined? SURELY YOU JEST...

[Ben Ocean]

Hi;
I have this code:
<dtml-if expr="URL[-(_.len('_frame')):]=='_frame'">
  <dtml-call "SESSION.set('select_blueline_temp',select_blueline(_.None,_))">
   <dtml-if expr="error_type == 'SessionNotFoundError'">
    <dtml-return happysession_exception>
   <dtml-else>
    <dtml-return index_frame>
   </dtml-if>
</dtml-if>
that produces a traceback stating *global error_type is not defined*. Huh? 
I thought this was part of the distro! What do?
TIA,
BenO