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I am trying to switch between TODO and DONE states with Shift-Right 
Arrow and I get a (wrong-type-argument stringp nil) error.

Please find attached the backtrace.


Emacs  : GNU Emacs 23.2.1 (i486-pc-linux-gnu, GTK+ Version 2.20.0)
  of 2010-12-11 on raven, modified by Debian
Package: Org-mode version 7.7

current state:
==============
(setq
  org-export-latex-after-initial-vars-hook '(org-beamer-after-initial-vars)
  org-speed-command-hook '(org-speed-command-default-hook 
org-babel-speed-command-hook)
  org-agenda-custom-commands '(("d" todo #("DELEGATED" 0 9 (face 
org-warning)) nil)
			      ("c" todo #("DONE|DEFERRED|CANCELLED" 0 23 (face org-warning)) nil)
			      ("w" todo #("WAITING" 0 7 (face org-warning)) nil)
			      ("W" agenda "" ((org-agenda-ndays 21)))
			      ("A" agenda ""
			       ((org-agenda-skip-function
				 (lambda nil
				  (org-agenda-skip-entry-if (quote notregexp) "\\=.*\\[#A\\]"))
				 )
				(org-agenda-ndays 1)
				(org-agenda-overriding-header "Today's Priority #A tasks: "))
			       )
			      ("u" alltodo ""
			       ((org-agenda-skip-function
				 (lambda nil
				  (org-agenda-skip-entry-if (quote scheduled) (quote deadline)
				   (quote regexp) "<[^>\n]+>")
				  )
				 )
				(org-agenda-overriding-header "Unscheduled TODO entries: "))
			       )
			      )
  org-agenda-files '("~/Docs/org/todo.org")
  org-metaup-hook '(org-babel-load-in-session-maybe)
  org-after-todo-state-change-hook '(org-clock-out-if-current)
  org-export-blocks-postblock-hook '(org-exp-res/src-name-cleanup)
  org-export-latex-format-toc-function 'org-export-latex-format-toc-default
  org-agenda-skip-scheduled-if-done t
  org-tab-first-hook '(org-hide-block-toggle-maybe 
org-src-native-tab-command-maybe
		      org-babel-hide-result-toggle-maybe)
  org-src-mode-hook '(org-src-babel-configure-edit-buffer 
org-src-mode-configure-edit-buffer)
  org-confirm-shell-link-function 'yes-or-no-p
  org-export-first-hook '(org-beamer-initialize-open-trackers)
  org-startup-indented 1
  org-agenda-before-write-hook '(org-agenda-add-entry-text)
  org-default-notes-file "~/Docs/org/notes.org"
  org-blank-before-new-entry nil
  org-babel-pre-tangle-hook '(save-buffer)
  org-cycle-hook '(org-cycle-hide-archived-subtrees 
org-cycle-hide-drawers org-cycle-show-empty-lines
		  org-optimize-window-after-visibility-change)
  org-export-preprocess-before-normalizing-links-hook 
'(org-remove-file-link-modifiers)
  org-mode-hook '((lambda nil
		  (org-add-hook (quote change-major-mode-hook) (quote org-show-block-all)
		   (quote append) (quote local))
		  )
		 (lambda nil
		  (org-add-hook (quote change-major-mode-hook) (quote 
org-babel-show-result-all)
		   (quote append) (quote local))
		  )
		 org-babel-result-hide-spec org-babel-hide-all-hashes)
  org-agenda-ndays 7
  org-refile-targets '((org-agenda-files :maxlevel . 5))
  org-ctrl-c-ctrl-c-hook '(org-babel-hash-at-point 
org-babel-execute-safely-maybe)
  org-cycle-separator-lines 1
  org-confirm-elisp-link-function 'yes-or-no-p
  org-fast-tag-selection-single-key 'expert
  org-agenda-start-on-weekday nil
  org-export-interblocks '((lob org-babel-exp-lob-one-liners) (src 
org-babel-exp-inline-src-blocks))
  org-clock-out-hook '(org-clock-remove-empty-clock-drawer)
  org-agenda-skip-deadline-if-done t
  org-reverse-note-order t
  org-occur-hook '(org-first-headline-recenter)
  org-from-is-user-regexp "\\<User\\>"
  org-export-preprocess-before-selecting-backend-code-hook 
'(org-beamer-select-beamer-code)
  org-tags-column 120
  org-remember-templates '(("Journal" 106 "* %U %? %^g\n\n   %x" 
"~/Dropbox/org/journal.org"
			   (quote top))
			  )
  org-export-latex-final-hook '(org-beamer-amend-header org-beamer-fix-toc
			       org-beamer-auto-fragile-frames
			       org-beamer-place-default-actions-for-lists)
  org-metadown-hook '(org-babel-pop-to-session-maybe)
  org-export-blocks '((src org-babel-exp-src-block nil) (comment 
org-export-blocks-format-comment t)
		     (ditaa org-export-blocks-format-ditaa nil) (dot 
org-export-blocks-format-dot nil))
  )