programming error: cannot align on self: empty element

[melkiy]

Hello.

I write choir scores with 4 voices on 3 staffs. Under the staffs i write
lyrics but not all the time: sometimes the lyrics is the same for two
staffs, and i want to leave the words between 2 staffs. That is i want the
lyrics to be under the 1st staff and not to be under the 2nd one somewhere -
not to overburden the scores. 

I tried to hide the identical part of text with
   \override LyricText   #'stencil = ##f %make text invisible
   \override LyricHyphen #'stencil = ##f %make text hyphens invisible
   some text
   \revert LyricText   #'stencil
   \revert LyricHyphen #'stencil

but this resulted in heaps of warnings: 
"programming error: cannot align on self: empty element
continuing, cross fingers".

I hope you would give me one of the workarounds:

1) explain why the warnings appear (although the scores are compilable)

2) tell me a better way to stop and to start lyrics for a certain voice when
i wish. The structure of my score is:

  \score 
  {
    \new ChoirStaff 
    <<
      \new Staff
      <<
        \clef "treble"
        \new Voice = "Sop"  { \voiceOne \TimeKey \SopMusic }
        \new Voice = "Alto" { \voiceTwo \AltoMusic }
        \new Lyrics \lyricsto "Sop" { \WordsSop }
      >>
      \new Staff
      <<
        \clef "treble"
        \new Voice = "Tenor" { \oneVoice \TimeKey \TenorMusic }
        \new Lyrics \lyricsto "Tenor" 
        {
              \WordsTenor  %!!!! This words need not to be continuously
        }
      >>
%Bass staff yet....
    >>
    \layout  { }
  }

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