NSUserNotificationCenter.defaultUserNotificationCenter () returns None in python

I am trying to connect to the Mountain Lion notification center via python. I installed pyobjc and follow the instructions here and here . Also see: Working with Mountain Lion Notification Center using PyObjC

Here is my code:

import Foundation, objc
import AppKit
import sys

NSUserNotification = objc.lookUpClass('NSUserNotification')
NSUserNotificationCenter = objc.lookUpClass('NSUserNotificationCenter')

def notify(title, subtitle, info_text, delay=0, sound=False, userInfo={}):
    """ Python method to show a desktop notification on Mountain Lion. Where:
        title: Title of notification
        subtitle: Subtitle of notification
        info_text: Informative text of notification
        delay: Delay (in seconds) before showing the notification
        sound: Play the default notification sound
        userInfo: a dictionary that can be used to handle clicks in your
                  app applicationDidFinishLaunching:aNotification method
    """
    notification = NSUserNotification.alloc().init()
    notification.setTitle_(title)
    notification.setSubtitle_(subtitle)
    notification.setInformativeText_(info_text)
    notification.setUserInfo_(userInfo)
    if sound:
        notification.setSoundName_("NSUserNotificationDefaultSoundName")
    notification.setDeliveryDate_(Foundation.NSDate.dateWithTimeInterval_sinceDate_(delay, Foundation.NSDate.date()))
    NSUserNotificationCenter.defaultUserNotificationCenter().scheduleNotification_(notification)

When I call the notification function with arguments, I get an attribute error:

AttributeError: 'NoneType' object has no attribute 'scheduleNotification_'

I do not understand why the NSUserNotificationCenter.defaultUserNotificationCenter()NoneType object returns. I could not ask for anything about this on the Internet or SO.