This has been happening for some time, and it's annoying enough that I want to fix it. With CMUCL 20b and slime 2010-09-20, try the following: (defvar *s* (make-string 4)) *s* (setf (lisp:codepoint *s* 0) #x10000)) Upto now, everything is ok. Now print the string: *s* At this point, the string is displayed, with a rectangular box for the codepoint #x10000 followed by two ^@ for the two null characters. (Recall that unicode strings in cmucl are utf-16 strings, so the first two elements of *s* are the surrogate pair for #x10000.) I get an error from xemacs about itimer: itimer "run-at-time<2>" signaled: (wrong-type-argument listp write-string) Wrong type argument: listp, :write-string And *slime-events* buffer is kind of garbled: (:emacs-rex (swank:listener-eval "(print *s*)\n") "COMMON-LISP-USER" :repl-thread 18) (:write-string "\n\"𐀀