Er, sorry, missed "(from JSONable combinations of types)". It's early.

Anyway, I can change the error message, but I will not make it special-
case single quotes for its own error message. I will have to think about 
what the message could say instead. Note that the same sort of person who 
throws random input at parsers might even expect {test: 'test'} to work 
since that is valid JavaScript but not valid JSON.