Error replacing %n escapes with nth %^{...} string in emacs org capture

Question

I am trying to write a custom org-capture template, but I get an error if I try to use the %\N functionality to insert the N-th prompt. The relevant excerpt of my org-capture-templates variable is

("ao" "Other meeting" entry
 (file+headline "~/org/meeting.org" "Meetings")
 "* %^{meeting date}u Meeting with %^{meeting with}
   :PROPERTIES:
   :MEETWITH: %\\2
   :DESCRIPTION: %\\3
   :END:

   Created on %U
   Description: %^{description}

   %?" :jump-to-captured t :empty-lines 1)

When I try to capture, everything goes fine until I C-c C-c the note. Then, the new note fails to save due to the following error:

org-capture: Capture abort: Wrong type argument: stringp, nil

This error only occurs when I try to use the %\N functionality, not otherwise. I assume the issue must be somehow related to the relevant section of org-capture-fill-template, but I am unable to discern what could be causing this errant behaviour. That section of code is as follows (from org-capture.el).

    ;; Replace %n escapes with nth %^{...} string.
    (setq strings (nreverse strings))
    (save-excursion
      (while (re-search-forward "%\\\\\\([1-9][0-9]*\\)" nil t)
        (unless (org-capture-escaped-%)
          (replace-match
           (nth (1- (string-to-number (match-string 1))) strings)
           nil t))))

Note: I have org-roam and org-roam-bibtex installed. Could this have anything to do with it?

Answer 1

Apparently, the %^{meeting date}u does not count as a prompted entry (perhaps because of the u? I'm not really sure). So it thinks there are only two prompted entries: %^{meeting with} and %^{description} so the template should look like this:

("ao" "Other meeting" entry
 (file+headline "~/org/meeting.org" "Meetings")
 "* %^{meeting date}u Meeting with %^{meeting with}
   :PROPERTIES:
   :MEETWITH: %\\1
   :DESCRIPTION: %\\2
   :END:

   Created on %U
   Description: %^{description}

   %?" :jump-to-captured t :empty-lines 1)

EDIT: I looked at the code (in org-capture.el:org-capture-fill-template) more carefully. The only time when prompted entries are saved for later use with %\N constructs is when they are not decorated with a key (CgGLptTuUptTuU). So the diagnosis above is correct: the u causes the entry not to be remembered for further use with %\N.

This seems deliberate so it's probably not a bug, but a design decision. In particular, the doc string for org-capture-templates says:

%\1 ... %\N Insert the text entered at the nth %^{prompt}, where N is a number, starting from 1.

Note that it does not mention the %^{prompt}<key> form at all.