Putting fret diagrams in a separate score

[Marc Hohl]

Hello list,

I try to put fret diagrams at the beginning of a piece.
My input file looks like this:

\version "2.13.50"

\score {
<<
    \new ChordNames \chordmode {
      g:m
    }
    \new Lyrics \lyricmode {
      \markup { \fret-diagram #"w:4;4-o;3-o;2-1;1-3;" }
    }
>>
  \layout { ragged-right = ##f }
}

\score {
<<
  \new Staff {
    \new Voice { c4 d e f }
  }
  \new Staff {
    \new Voice { c,4 d, e, f }
  }
>>
}

It compiles with lots of messages

Warning: staff-affinities  should only decrease

but the output looks fine. Why do these errors appear?
(I used this technique several years before, and I think it compiled 
without errors,
but I am not sure and of course didn't save the log output ;-)

Regards,

Marc