PianoStaff, multiple pages spacing issue

[Alexander Rose]

Hey,


if you have a piano score that is bigger than one page, the music on  
all pages but the last is distributed over a page by extending the  
space between the two staffs of the PianoStaff instead of extending  
the space between the lines of the PianoStaff.

There is a warning: "Warnung: Can't fit systems on page -- ignoring  
between-system-padding"

\version "2.11.17"

upper = \relative c'' {
  \repeat unfold 60 { c4 c4 c4 c4 }
}

lower = \relative c'' {
  \repeat unfold 60 { c4 c4 c4 c4 }
}


\score {
  \new PianoStaff
  << \set PianoStaff.instrumentName = "Piano  "
     \new Staff = "upper" \upper
     \new Staff = "lower" \lower
   >>
}