Lyric alignment problem with custom Voice

[Roman Stawski]

This short polyphony employs
A trivial customised Voice
  but the lyrics ignore
  the first note in the score --
that's not the behaviour of choice.

-----
\version "2.13.11"
\layout { ragged-right = ##t }

\layout {
  \context {
    \Voice
    \name MainVoice
    \alias Voice
  }
  \context {
    \Staff
    \accepts "MainVoice"
  }
}

tx = \lyricmode   { Foo bar }
va = \relative c' { c2  g'2 }
vb = \relative c' { a2  c2  }

<<
  % First note ignored by lyrics :-(
  \new Staff <<
    \new MainVoice = "ta" \va \\
    \vb
  >>
  \new Lyrics { \lyricsto "ta" \tx }

  % Compare with ...
  \new Staff <<
    \new Voice = "tb" \va \\
    \vb
  >>
  \new Lyrics { \lyricsto "tb" \tx }
>>
-----

Roman (with apologies for the doggerel)