Lyrics assignment problem

[Bertalan Fodor]

Hello, 

I have a score where an instrument and singer are answering each other.
I thought it would be natural to use 
\context Voice = instrument { ... }
\context Voice = singer { ... }
\context Voice = instrument { ... }
\context Voice = singer { ... }
\lyricsto "singer" { la -- la -- la }

But lyricsto only assigns the lyrics to the first occurence of singer. It is
strange that if I say 
\lyricsto "instrument" 
the lyrics is assigned to all occurences.

Lily version: 2.2.2

Source attachment:

\version "2.2.2"

\score {
        <<
        \new Staff \notes \relative c' {
                \context Voice = instrument  { c8 d e d c2 }
                \context Voice = singer { e8 f g f e2 }
                \context Voice = instrument { g8 f e d e2 }
                \context Voice = singer { e8 d c b c2 }
                \context Voice = instrument { d8 d e d c2 }
                \context Voice = singer { d8 d e d c2 }
        }
        \lyricsto "singer" \new Lyrics \lyrics { Pam -- pam -- pam -- pam --
pam. 
                La -- la -- la -- la -- la. 
                Blah -- blah -- blah -- blah -- blah. 
        }       
        >>
}

Thanks,

Bert