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