staccato overlaping on staff line

[Karol Majewski]

\version "2.19.3"

upper = {
  \clef treble \time 2/4
  \voiceOne a''8 \noBeam <b' d'' g''>8
}

lower = {
  \clef bass \time 2/4
  \override Beam.auto-knee-gap = ##f
  \voiceTwo b8 -. \change Staff = "upper" <d' e' g'>8 -.
}

\score {
  \new PianoStaff <<
    \new Staff = "upper" \upper
    \new Staff = "lower" \lower
  >>
}

overlap.png
Description: PNG image