Hello. Huncentoot used md5 library to generate session strings. The function MD5:MD5SUM-SEQUENCE allows only SIMPLE-STRINGs as arguments. Hunchentoot passes result of FORMAT, which is not necessary returns SIMPLE-STRING (although in many implementations it does). In particular just encountered this error when testing Hunchentoot on ECL (with the patch sent by Marko Kocic here: http://common-lisp.net/pipermail/tbnl-devel/2008-November/004474.html) The fix will be as follows: (defun md5-hex (string) "Calculates the md5 sum of the string STRING and returns it as a hex string." (with-output-to-string (s) - (loop for code across (md5:md5sum-sequence string) + (loop for code across (md5:md5sum-sequence (coerce string 'simple-string)) do (format s "~2,'0x" code)))) Best regards, - Anton