Oops when adding event to calendar

OOPS-B35:
...
URL: https://launchpad.net/calendar/+add
...
  Module zope.tales.tales, line 698, in evaluate
    return expression(self)
   - /srv/launchpad.net/production/launchpad/lib/canonical/launchpad/templates/l
aunchpad-addform.pt
   - Line 38, Column 8
   - Expression: <PathExpr standard:'view/update'>
   - Names:
      {'args': (),
       'context': <canonical.launchpad.components.cal.MergedCalendar object at 0
x2aa8ed67d0>,
       'default': <object object at 0x2a955c01d0>,
       'loop': {},
       'nothing': None,
       'options': {},
       'repeat': {},
       'request': <canonical.launchpad.webapp.servers.LaunchpadBrowserRequest in
stance URL=https://launchpad.net/calendar/+add>,
       'template': <zope.app.pagetemplate.viewpagetemplatefile.ViewPageTemplateF
ile object at 0x2a9f6a03d0>,
       'usage': <zope.pagetemplate.pagetemplate.TemplateUsage object at 0x7656cd
0>,
       'view': <zope.app.pagetemplate.simpleviewclass.SimpleViewClass from /srv/
launchpad.net/production/launchpad/lib/canonical/launchpad/templates/calendar-ev
ent-addform.pt object at 0x2aa7867b90>,
       'views': <zope.app.pagetemplate.viewpagetemplatefile.ViewMapper object at
 0x7656c90>}
  Module zope.tales.expressions, line 204, in __call__
    return self._eval(econtext)
  Module zope.tales.expressions, line 198, in _eval
    return ob()
  Module zope.app.form.browser.add, line 59, in update
    self.createAndAdd(data)
  Module canonical.launchpad.browser.cal, line 740, in createAndAdd
    dtstart = event.dtstart.astimezone(
AttributeError: 'NoneType' object has no attribute 'dtstart'