I'm using release_8.2.10 and experienced the same problem.
The problem seems to be the function org-agenda-fontify-priorities which
calls:
(org-face-from-face-or-color
'priority nil
(cdr (assoc p org-priority-faces)))
which expects a face to inherit from. When org-priority-faces is nil or
color or even does not specify the priority(e.g. "B") it inherits from
nil and causes the message.
Setting it to something like
(quote
((65 . org-level-1)
(66 . org-level-2)
(67 . org-level-3)
(68 . org-level-4))))
is a workaround to the problem.
Renato
