Positioning tuplet numbers close to kneed beams not working

[Nikolai Ho]

Hello,
I cannot get the tuplet number to appear directly above the kneed bar. What am 
I doing wrong? Is it perhaps the version I am using? Thank you!



\version "2.18.2"

upper =  
  \relative c' {
  \clef treble
  \key d \major
  %%%...………………………………
}

lower = 
  \relative c {
  \clef bass
  \key d \major
  \time 6/8     

  %%%……………………………………..

  \override Beam.auto-knee-gap = #1 %%%%%%%%%%%%NOT WORKING

  \tuplet 10/8 {\stemDown\change Staff = "upper" \crossStaff a'32 e cis 
\stemUp\change Staff = "lower" \crossStaff gis a, e a gis'                      
                     \stemDown\change Staff = "upper" \crossStaff cis e}

  \tuplet 10/8 {a e cis \stemUp\change Staff = "lower" \crossStaff gis a, e a  
gis' \stemDown\change Staff = "upper" \crossStaff cis e}  \tuplet    10/8 {a  e 
cis \stemUp\change Staff = "lower" \crossStaff gis a, e a gis' \stemDown\change 
Staff = "upper" \crossStaff cis e} \tuplet 10/8 {b'   e, cis \stemUp\change 
Staff = "lower" \crossStaff gis a, e a gis' \stemDown\change Staff = "upper" 
\crossStaff cis e} \tuplet 10/8 {cis' gis e   \stemUp\change Staff = "lower" 
\crossStaff a, a, e a a' \stemDown\change Staff = "upper" \crossStaff e' gis}
}

\score {
  \new PianoStaff <<
    \new Staff = "upper" \upper
    \new Staff = "lower" \lower    
    >>
  \layout {
    \context {
      \PianoStaff
      \consists #Span_stem_engraver
    }
    \context {
      \Staff
    }
  }
  \midi {}
}