regression: assertion

[lilypond]

Code below works fine when run under windows in lilypond 2.18.2
However when running in 2.19.83, or in 2.21.0 (version end November) an
assertion is thrown:

File:
/home/jaap/gub/target/mingw/src/lilypond-git.sv.gnu.org--lilypond.git-master
/lily/page-breaking.cc, Line 1052
Expression: my_index == 0

\version "2.18.2"

\header {
    title = "Title"
    }

\relative fis' { 
  \pageBreak
  r1 \break
  r4 r4 r2
  r2 r4 c4 \break
  r1 \pageBreak
  r1 
}