polyphony & placement of accidentals

[Luc]

In this measure under Windows XP, the notehead of the last "bes" is  
separated from the accidental by the upper voice - can this be remedied?

\version "2.8.6"

\score {
 <<
  \override Staff.NoteCollision  #'merge-differently-headed = ##t
  \override Staff.NoteCollision  #'merge-differently-dotted = ##t
  #(set-accidental-style 'modern)

  \context Voice="oben" {
   \stemUp
   \clef "G"
   \key  f \major
   \time 3/8
    g       8
    c   ''  16
    b   '   16
    c   ''   8
  }
  \\
  \context Voice="unten" {
   \stemDown
    g       8
    bes '     8
    bes '     8
  }
 >>
 \layout {ragged-right= ##t}
}