This is an error, something I overlooked after Shaun added the new Aggregated Forum type to the forums.
To let you know, my intention is to never allow someone to subscribe to the Aggregated Forum, thus always hiding that. The reason is the way this is stored in the db is by forumid and thats why you see the error. I am not sure that is the best overall solution but that is a good fix that can be done with minimal effort.
I have to spend some serious time looking at the entire notification/email setup in this module. I am also weighing how I can utilize the core along w/ scheduled tasks to handle this in a background thread too. Because the amount of effot I need to spend to architect this as well as implement it, this is something that is going to take a long time to implement properly.
Another side of this is why should someone be able to subscribe to the aggregated view. All the aggregated view is, simply put, is all the non-private forums rolled into a single one. Would a notify all be a better option?
I definately invite some feedback on the items around this. I hope the community sees that tons of thought is put into these things and that even though it may not seem that way, this module is driven based on the community feedback. It can be very difficult at times deciding what is best codewise/architectually as most developers know. So keep that in consideration but also consider the majority of use cases when when determining direction then factor in availability of the teams minimal resources and that is basically the fomula I deal with when deciding where to go with the module. Anyways, my rambling aside, please speak out what you think is best in this situation.