Segfault with voice and empty music

[Roman Stawski]

Hi folks

I've noticed the following problem in 2.19.16 after updating from 2.19.11.

When the code below compiles I get the error
    "warning: cannot find Voice `refrain"
and then the  compilation segfaults.

If I comment out the line marked "NOT OK" then the compilation
and the output are fine.

It seems that the empty music and the voices don't play well
together inside the inner double brackets. Any ideas, anyone?

(Duh: The reason that I'm generating empty music is that I'm
 conditionally inserting other lyrics in the real piece.)

Roman

---- Problem code follows

\version "2.19.16"

<<
  \new Staff {
    \new Voice = "couplet" \relative c' { \voiceOne c2 }
    \new Voice = "refrain" \relative c' { \voiceOne c2 }
  }

  #(make-music 'Music)  %% OK
  \tag #'optional <<
    #(make-music 'Music) %% NOT OK
    \new Lyrics \lyricsto "couplet" { Foo }
    \new Lyrics \lyricsto "refrain" { Bar }
  >>
  #(make-music 'Music)  %% OK
>>