[Zope] dtml-calendar: cannot update existing entry

[Daniel Huang]

I'm following Jon Udell's tutorial 
(http://zope.org/Members/judell/CalendarTagExample) verbatim and have 
successfully created entries in the calendar (version 1.0.15) .  However 
when I click on an existing entry and try to change the content, I get the 
following error:

Error Type: Bad Request
Error Value: Invalid or duplicate property id.


Its happening in the editCalendarProperty code:
///////////////
REQUEST = context.REQUEST

if ( context.hasProperty(REQUEST['prop']) ):
     context.manage_changeProperties({ REQUEST['prop'] : REQUEST['propval'] })
else:
     context.manage_addProperty(REQUEST['prop'], REQUEST['propval'], 'string')
return REQUEST.RESPONSE.redirect(REQUEST['BASE4'])
////////////////

I think "context.hasProperty" always returns false and so it tries to add a 
duplicate property.  But I dont see why.  Has anyone run into the same issue?