Error with color-segment function but only when certain faces returned

[Cerebus]

OK, I give up.

(setq telephone-line-faces
   '((modified . my/modified-face-fun)))

(setq telephone-line-lhs '((modified . (my/status-segment))))

(telephone-line-defsegment* my/status-segment ()
    `(""
      mode-line-modified))

(defun my/modified-face-fun (active)
    (cond ((not active) 'mode-line-inactive)
	  ((buffer-modified-p) 'error)
	  (t 'mode-line)))

Errors:

Error during redisplay: (eval (telephone-line-add-subseparators '(#[(face) ('("" mode-line-modified)) (t)]) telephone-line-identity-hollow-left 'modified) t) signaled (wrong-type-argument number-or-marker-p nil) [3 times]

Yet if I return ((buffer-modified-p) 'default) instead, it works. error is clearly a face; I can see it in the faces list and I can customize it. Also errors returning other lots of other faces...but not everything.