Wouldn't mind seeing a slightly more explicit exception here.  Maybe `IllegalStateException`?  Or `ex-info` like I did with the prismatic plumbing PR?  I know that you guys decided you didn't always want to use slingshot for stuff like this, but it's not entirely out of the realm of possibility that someone might try to catch this... after all, it's not entirely dissimilar to what we did with prismatic.  They obviously weren't thinking that people were going to want to catch their exceptions, but we came up with a use case for it.

I don't have any particularly strong feelings here, just a thought.