What steps will reproduce the problem?
1. orgmode 0.34c, emacs 23 on Ubuntu 9.10 (this function used to work in previous
versions of orgmode)
2. Write an org file
3. Use the provided function to export as twiki to find the error message


What is the expected output? What do you see instead?
Here is the backtrace:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\[\\[\\([^][]+\\)\\]\\(\\[\\([^][]+\\)\\]\\)?\\]\\|[ 

]+\\(:[[:alnum:]_@:]+:\\)[ 	]*$" nil)
  org-docbook-expand(nil)
  byte-code("
  org-export-as-docbook()
  (let* ((wconfig ...) (docbook-buf ...) (filename ...) (base ...)
(twiki-file ...)) (and (file-exists-p twiki-file) (delete-file twiki-file))

(message "Processing DocBook XML file...") (shell-command (format "xsltproc
--output %s /home/mariomu/synchronize/site-lisp/docbook2twiki.xsl %s"
twiki-file ...)) (message "Processing DocBook file...done") (if (not ...)

(error "TWiki file was not produced") (set-window-configuration wconfig)
(message "Exporting to TWiki...done") twiki-file))
  org-export-as-twiki()
  call-interactively(org-export-as-twiki nil nil)