[I added emacs-orgmode@gnu.org in the Cc:]

On Mon, 03 May 2021 18:07:25 +0200 Stephen Berman <stephen.berman@gmx.net> wrote:

> By default `i' is a prefix key in calendar-mode for commands that insert
> diary entries.  But if you happen to display a buffer that activates
> org-mode machinery, then `i' in calendar-mode becomes bound to
> org-agenda-diary-entry and typing it can raise a wrong-type-argument
> error.  This can happen by visiting a file in Org mode.  To reproduce:
>
> 0. emacs -Q
> 1. (sanity check:) Type `M-x calendar RET' and then in the Calendar
>    buffer type `i C-h': the *Help* buffer displays all the commands
>    invoked by `i' plus one or more keys.
> 2. Visit the file `ORG-NEWS' (e.g. by typing `C-h n C-x C-f O TAB RET').
> 3. Type `M-x calendar RET' and then in the Calendar buffer type `i'
> => Wrong type argument: commandp, org-agenda-diary-entry
>
> This can also catch users by surprise, e.g. in Gnus.  To reproduce,
> replace step 2 above by the following:
>
> 2a. Type `M-x gnus', answer `y' at the prompt; in the Gnus buffer type
>     `B RET news.gmane.io RET'.
> 2b. In the *Gnus Browse Server* buffer type `C-s humani' to put point on
>     the gmane.emacs.humanities group; type RET to enter it.
> 2c. Type `j <87sg6wulu6.fsf@localhost> RET', which displays an article
>     containing an org-mode source code block.
> 3. As above, resulting in the same error (when done from emacs -Q).
>
> The Org mode manual (info "(org) Agenda Commands") does describe its use
> of the `i' binding in the Calendar, and if Org mode has its own versions
> of the commands that use `i' by default in calendar-mode, then
> overriding the calendar-mode bindings is no problem for Org Agenda
> users, but those bindings should not be overridden just by displaying a
> buffer that happens to be in org-mode or happens to contain an Org
> source code block.

The following patch fixes the problem for me: