When attempting to use the new sorting methods (like ~scheduled-up~), I
receive the following error messages when displaying the agenda (with
=M-x org-agenda-list= or ~C-a a a~):

: org-entries-lessp: Wrong type argument: stringp, nil

To replicate this issue, try taking following steps with Emacs 23.4:

: $ emacs -Q (some-file.org)
: (add-to-list 'load-path "/usr/share/emacs/site-lisp/org-8.0/lisp")
: (add-to-list 'load-path "/usr/share/emacs/site-lisp/org-8.0/contrib/lisp" 't)
: (require 'org)
: M-x revert-file
: M-x org-agenda-list
: M-x customize-group org-agenda-sorting
: (Change sorting strategy: "time-up" becomes "scheduled-up")
: (Save settings)
: M-x org-agenda-list

When displaying the list, you'll get this output:

: org-entries-lessp: Wrong type argument: stringp, nil

Can anyone else reproduce this issue?  What am I missing here?

Thanks for your time,
Nick