Lyrics interfere with volta?

[Colin Campbell]

> not top posting
I'm setting a vocal piece containing a simple repeat. The code below shows
problems with prematurely ending a volta, but only if the Lyrics block is
included. I've trimmed the live version down to this:
%<---------------------------------------------------------
\version "2.17.14"
\language "english"

\header {
  title = "Lyric volta bug"
}

<<
  \new Staff  {
    \new Voice = "sop" {
      \repeat volta 2 { ef'1 }
      \alternative {
        { r2 r4 r8 bf8  }
        { r2 r4 r8 bf8  }
      }
      ef'4-. r4 r2
    }
  }
  
  \new Lyrics { \lyricsto "sop" { 
    \repeat volta 2 { gu. }
    \alternative {
      {  Mun -- }
      {  Mun -- }
    }
    gu.
                }
  }
>>
%<-------------------------------------------------

I believe I've read the Known Issues on ties, slurs and grace notes but this
seems too simple for any of those. If RTFM is indeed the answer, my pride will
gratefully take second place to my sanity!

Cheers,
Colin