Can't assign LocalNotification.userInfo?

I have a pretty straightforward piece of code:

 LocalNotification notification = new LocalNotification();
 IDictionary userInfo = new Dictionary<string, string>(1);
 userInfo["Type"] = notificationType.ToString();
 notification.userInfo = userInfo;

Immediately after this, I'd expect (notification.userInfo == userInfo) to be true... but it is not! In fact, notification.userInfo is NULL even though the local variable userInfo is NON-NULL.

WTF. O_o