Django's internal foreign key does not match the parent primary key

I need help to get past django error using inline forms. I can not understand this. Perhaps I could insert an event identifier if I could figure out how to do this before he tries to check.

When I submit my form, I get an error: Hidden field (event): The inline foreign key did not match the parent instance primary key

Is there any way to make this code work?

The following code should allow the teacher to enroll several students for the event.

------ models.py ------------

class Event(models.Model):
    title = models.CharField(max_length=200)
    description = models.CharField(max_length=200)
    time = models.DateTimeField()
    duration = models.DecimalField(max_digits=5, decimal_places=2)
    location = models.CharField(max_length=200)


# Customer is a parent, teacher, or school
class Customer(models.Model):
    event = models.ForeignKey(Event)
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30) 
    address1 = models.CharField(max_length=60) 
    address2 = models.CharField(max_length=60, blank=True) 
    city = models.CharField(max_length=30) 
    state = models.CharField(max_length=2) 
    zipcode = models.CharField(max_length=5) 
    phone_number = models.CharField(max_length=30)
    email = models.EmailField()


# A customer can enroll several students for a single event.
class Attendee(models.Model):
    event = models.ForeignKey(Event)
    sponsor = models.ForeignKey(Customer)
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30) 

------ forms.py -----------

class AttendeeForm(forms.ModelForm):
  event = forms.IntegerField(required=True, widget=forms.HiddenInput())

  class Meta:
    model = Attendee
    fields = ( 'event', 'last_name', 'first_name', 'gender', 'schoolYr', )
    #exclude = ('event', )
    #widgets = {
    #     'dtgPurchase' : DateTimeWidget(),
    #}

  def __init__(self, *args, **kwargs):
      super(AttendeeForm, self).__init__(*args, **kwargs)

------- views.py ----------

def register3(request, event_id):
  messages = []
  try:
    event = get_object_or_404(Event, id=event_id)  #Event ID is passed in here.
    AttendeeFormSet = inlineformset_factory(Event, Attendee, form=AttendeeForm, extra=1)
    #AttendeeFormSet = formset_factory(AttendeeForm)
    if request.method == 'POST':
      print "POST"
      formset = AttendeeFormSet(request.POST, request.FILES, prefix='attendees')
      #formset.save(commit=False)
      i=0
      for form in formset.forms:
         print "Form %s " % str(i)
         #  ??  Is is possible to set the event Here? 

      if formset.is_valid():
          attendees = formset.save_all()
          print "Yay!!!"   
          #return redirect('event_view', event_id=event.id)
      else:
          print "Invalid formset"

    else:   # is get method on first step
      formset = AttendeeFormSet(instance=event, prefix='attendees')
      print "Event: %s " % event

  except Event.DoesNotExist:
      raise Http404

  c = Context({
      'messages': messages,
      'event' : event,
      'attendees': formset,
  })
  return prepCxt(request, 'register3.html', c)