ClojureScript doesn't handle printing objects with a null prototype:
(prn (.create js/Object nil)) ;;throws Uncaught TypeError: Cannot read property 'cljs$lang$ctorStr' of undefined
(str (.create js/Object nil)) ;;throws Uncaught TypeError: Cannot convert object to primitive value
For the first case, it looks like pr-writer-impl's last case needs a separate check for a nil object constructor. I've not yet investigated the second case.