\new Staff { \key c \minor < es' g' > < bes' d''~ >2. | \tuplet 3/2 \voices "",2 << { \voiceOne d''4 c'' es'' \oneVoice } \\ { g'4 as'2 } >> < f' as' >4 <es' g'> | }
Admittedly, this is still better than the warning and crash an earlier version rather than current master puts out.
But now there is neither warning nor crash. -- David Kastrup