Why does `(window-line-height)' return nil?

[Xue Fuqiao <xfq.free@gmail.com>]

Hi,

In my *scratch* buffer, all of the following expressions return nil with
`eval-print-last-sexp' (also tried `eval-last-sexp'):

--8<---------------cut here---------------start------------->8---
(window-line-height 'header-line)
(window-line-height 'mode-line)
(window-line-height)
(window-line-height 0)
(window-line-height -1)
--8<---------------cut here---------------end--------------->8---

Why do they return nil?  After reading the doc string, I only found one
sentence that may be relavant: "Return nil if window display is not
up-to-date."  So, does that mean window display is not up-to-date when
evaluating the expression?  I've searched the archives/web, but found
little information about this function.

-- 
Best regards, Xue Fuqiao.
http://www.gnu.org/software/emacs/