In the case of numeric priorities [#1] [#9] and so on, there is a test
that is reversed in org.el.  This appears twice with a slight variation.

;; Are we are less than the highest or greater than the lowest?
(or (< (upcase new) org-priority-highest)
    (> (upcase new) org-priority-lowest))

The test is, in itself (and in principle) just fine.  The problem is
that it then triggers an error, which is exactly the opposite of what’s
wanted!

The attached patch reverses the test so that we enter the error case
when the priority is *outside* of the range of acceptable priority tags.

Here’s some test data for reproducing the problem and testing the
solution (with the patch applied).

Evaluate:

(setq org-default-priority 2)
(setq org-highest-priority 9)
(setq org-lowest-priority 1)

* This is a test
(Run M-x org-priority RET on the above line.)