Looks good. I did something like that when adapting the tests for
Gauche (except that I did "(eqv? #f (string-.." which is not as good
as "(not (string-...").
But would you mind investigating why it doesn't work on Chibi despite my patch? It still works on Larceny, but I don't want to ask Will until it works on Chibi, which enforces the types.