Rejected events

I just implemented GameAnalytics and while things seem to be generally working, I am getting lots of these in my dashboard:

Event rejected: Fields build,user_id and session_id are required for the ‘design’ category.

Anyone know why I’m getting these?  

I’m not explicitly passing build, user_id and session_id params when I send design events, but I’m under the impression that I shouldn’t need to. Even the example custom events in the docs do not pass these params.