Tremolo and partcombiner: core dump (assertion failure)

[Rutger Hofman]

Good afternoon list,

the following combination of \partcombine and \tremolo gives a core dump 
with the following error message:

Preprocessing graphical objects...lilypond: 
/home/gub/NewGub/gub/target/linux-64/src/lilypond-git.sv.gnu.org--lilypond.git-stable-2.20/flower/include/interval.hh:227: 
T Interval_t<T>::center() const [with T = double]: Assertion `!is_empty 
()' failed.

If the % \partcombineApart below is uncommented, there is no core dump.

Lilypond:

\version "2.19.80"

upp = \relative c'' {
    \repeat tremolo 32 { d64( b) } |
    d1 |
    b'1:64 |
}

low = \relative c'' {
    % \partcombineApart
    b1 |
    a1 |
    g1:64 |
}

\score {
    <<
        \new Staff \with {
            instrumentName = "upp"
        } <<
            \upp
        >>
        \new Staff \with {
            instrumentName = "low"
        } <<
            \low
        >>
        \new Staff \with {
            instrumentName = "partcombined"
        } <<
            \partcombine
                \upp
                \low
        >>
    >>
}

Rutger Hofman
Amsterdam