Django built-in signals problem: error when using post_save

I'm building an app that notifies user when new ThreadedComments appear. For this I'm using post_save signal. Here's my models.py:

from django.db import models
from dja开发者_如何学Pythonngo.contrib.auth.models import User
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic
from datetime import datetime

from threadedcomments.models import ThreadedComment
from django.db.models.signals import post_save

from blog.models import Post
from topics.models import Topic

class BuzzEvent(models.Model):
    user = models.ForeignKey(User)
    time = models.DateTimeField(default=datetime.now)

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey()

    def __unicode__(self):
        return self.content_object

def buzz_on_comment(sender, **kwargs):
    # This is called when there is a new ThreadedComment
    comment = kwargs['instance']

    user_attr_names = {
                  'post'    : 'author',
                  'topic'   : 'creator',
                  'tribe'   : 'creator',
                 }

    user = getattr(comment.content_object, 
                   user_attr_names[comment.content_type.model])

    BuzzEvent.objects.create(content_object=sender,
                             user=user,
                             time=datetime.now())

post_save.connect(buzz_on_comment, sender=ThreadedComment)

The problem is when a new ThreadedComment is created, I get an error: unbound method _get_pk_val() must be called with ThreadedComment instance as first argument (got nothing instead). Traceback and debugger says it happens when creating BuzzEvent object calls signals.pre_init.send. I'm unable to hack django code right now, is there any obvious solution or idea?

Looks like it should be model instance (which comment is) instead of model class (which sender is) as the content_object argument:

BuzzEvent.objects.create(content_object=comment,
                         user=user)