*** ASSERTION-VIOLATION: s is supposed to be of type <string>, but got #f
Stack Trace:
_______________________________________
  0  thunk
  1  (proc out)
  2  (call-with-output-string (cut with-error-to-port <> (cut repo ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":45
  3  (html-escape-string (call-with-output-string (cut with-error- ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":44
  4  (html:pre (html-escape-string (call-with-output-string (cut w ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":43
  5  (on-error e)
        at "/home/shiro/share/gauche-0.98/0.9.16_pre3/lib/www/cgi.scm":407
  6  (string-cursor-end s)
  7  (string-trim-both nick)
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":35
  8  thunk
  9  (call-with-output-file +current-file+ (cut update <> (string- ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":33
 10  thunk
 11  (chaton-with-exclusive-locking (^ () (call-with-output-file + ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":31
 12  (proc params)
        at "/home/shiro/share/gauche-0.98/0.9.16_pre3/lib/www/cgi.scm":410
 13  (with-error-handler (lambda (e) (cond (else (output-proc (on- ...
        at "libmacro.scm":525
        expanded from (guard (e (else (output-proc (on-error e)))) (let1 pa ...
        at "/home/shiro/share/gauche-0.98/0.9.16_pre3/lib/www/cgi.scm":407
 14  (cgi-main (^ (params) (let ((nick (cgi-get-parameter "nick" p ...
        at "/home/www/sites/chaton.practical-scheme.net/chaton/chaton-poster-chaton":23
 15  (main args)