cannot find Voice

[Ferenc Wagner]

Hi!

I get
warning: cannot find Voice: choir
when compiling the attached file.  The output nevertheless
seems good.  The warning disappears if I comment out the
\set command.  Where should I put it to avoid warnings?

\version "2.2.1"
\score {
    <<
        \new Staff {
            \set Staff.midiInstrument = "choir aahs"
            \context Voice = "choir" \notes { c' d' }
        }
        \lyricsto "choir" \new Lyrics \lyrics { du dumm }
    >>
}

Btw. '\lyricsto "choir"' and '\new Lyrics' seem to commute.
Not a real problem, but makes building a mental model of the
LilyPond language harder.  For me at least.  What's going on?
-- 
Thanks,
Feri.