Hello,
I tried to configure org-agenda-custom-commands by

(setq org-agenda-custom-commands
      '((" " "Agenda"
         ((tags-todo "/!+TODO"
                     ((org-agenda-overriding-header "University")
                      (org-agenda-todo-ignore-scheduled t)
                      (org-agenda-todo-ignore-deadlines t)
                      (org-agenda-todo-ignore-with-date t)
                      ;TODO: BUG, Indenting todo items does not work.
                      (org-agenda-prefix-format " %i %-12:c%l%s")
                      (org-agenda-sorting-strategy
                       '(category-keep)))))
         nil)))

I want the todo items to be indented according to their level with '%l'.
However, when I add the '%l' flag and try to open this agenda view, I
get:

concat: Wrong type argument: buffer-or-string-p, 2

The error can be reproduced with a clean config (emacs -Q -l
org-minimal.el) if one agenda file with todo items that match the
criteria is present.

Regards,
Dominik

Emacs  : GNU Emacs 24.4.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.7)
 of 2015-01-17 on bisson
Package: Org-mode version 8.3beta (release_8.3beta-1025-g7b97b6 @
/home/dominik/.emacs.d/org-mode/lisp/)

current state:
==============
(setq
 org-tab-first-hook '(org-hide-block-toggle-maybe
org-babel-hide-result-toggle-maybe
              org-babel-header-arg-expand)
 org-speed-command-hook '(org-speed-command-default-hook
org-babel-speed-command-hook)
 org-occur-hook '(org-first-headline-recenter)
 org-metaup-hook '(org-babel-load-in-session-maybe)
 org-confirm-shell-link-function 'yes-or-no-p
 org-agenda-custom-commands '((" " "Agenda"
                   ((tags-todo "/!+TODO"
                 ((org-agenda-overriding-header "University")
                  (org-agenda-todo-ignore-scheduled t)
                  (org-agenda-todo-ignore-deadlines t)
                  (org-agenda-todo-ignore-with-date t)
                  (org-agenda-prefix-format " %i %-12:c%l%s")
                  (org-agenda-sorting-strategy (quote (category-keep))))
                 )
                nil)
                   )
                  )
 org-after-todo-state-change-hook '(org-clock-out-if-current)
 org-src-mode-hook '(org-src-babel-configure-edit-buffer
org-src-mode-configure-edit-buffer)
 org-agenda-before-write-hook '(org-agenda-add-entry-text)
 org-babel-pre-tangle-hook '(save-buffer)
 org-mode-hook '(#[nil "\300\301\302\303\304$\207"
           [org-add-hook change-major-mode-hook org-show-block-all append
local] 5]
         #[nil "\300\301\302\303\304$\207"
           [org-add-hook change-major-mode-hook org-babel-show-result-all
append local] 5]
         org-babel-result-hide-spec org-babel-hide-all-hashes)
 org-archive-hook '(org-attach-archive-delete-maybe)
 org-ctrl-c-ctrl-c-hook '(org-babel-hash-at-point
org-babel-execute-safely-maybe)
 org-cycle-hook '(org-cycle-hide-archived-subtrees org-cycle-hide-drawers
org-cycle-show-empty-lines
          org-optimize-window-after-visibility-change)
 org-confirm-elisp-link-function 'yes-or-no-p
 org-metadown-hook '(org-babel-pop-to-session-maybe)
 org-agenda-files '("~/Shared/emacs/org-files/index.org"
"~/Shared/emacs/org-files/university.org")
 org-clock-out-hook '(org-clock-remove-empty-clock-drawer)
 )