Voicename not recognised in music function

[Marten Visser]

> I'm not top posting.

A voicename not is recognised in a music function.

In the example below, I'd expect to get two documents with the same 
content. However, in the "error" document, no lyrics are typeset, and 
an error is sent into the logfile. Why?

Thanks for looking,
--Marten

----------------------------------------
%{
Welcome to LilyPond
===================
** Unrecognised voice error **
Error occurs when calling music function typesetting lyrics.
%}

\version "2.13.32"

\book {
        \bookOutputSuffix "Okay"
        \score {
                <<
                        \new Voice = "melody" { \relative c'' { d cis b a } }
                        \new Lyrics     \lyricsto "melody" \lyricmode { An ex 
-- am -- ple. }
                >>
                \layout { }
        }
}

MusFunc = #(define-music-function (parser location) ()
        #{
                \new Voice = "melody" { \relative c'' { d cis b a } }
                \new Lyrics     \lyricsto "melody" \lyricmode { An ex -- am -- 
ple. }
        #} )

\book {
        \bookOutputSuffix "Error"
        \score {
                <<
                        \MusFunc
                >>
                \layout { }
        }
}