Can't Get Valid RSS Feed for Events +

I am having trouble getting a valid RSS feed for my events + calendar. I read a bunch of previous posts and tried the following (.../feed/?post_type=incsub_event&event_monthnum=12), but tried using it with other services (such as Mail Chimp) and it says it's not a valid feed. I've also checked with other feed sources such as W3 Validator and they same the same thing. When you go to the address it shows all of the events, but it's not a valid feed. Please advise.