The reason the message is so cryptic is that if Snap! has an error that we're not anticipating, you get the JavaScript error message, which isn't often related to what the user is trying to do.
Jens is proud of Snap!'s ability to let you modify a program while it's running. So he might decide to fix this expeditiously. I'm more inclined to say "Just don't relabel Control blocks while the program is running!" :~)