g:actual_curwin undefined

I want to use g:statusline_winid and g:actual_curwin to detect
whether the currently drawn status line is the active one.

With Vim 8.1.1517, I have tried the following code:

fun! BuildStatusLine()
return g:statusline_winid ==# g:actual_curwin
\ ? 'Active' : 'Inactive'
endf

set statusline=%!BuildStatusLine()

but I get E121: Undefined variable: g:actual_curwin.

My understanding is that the global variables should be defined
when a %! expression is evaluated. Is that correct? If so, what
am I doing wrong?

Thanks,
Life.

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