crossStaff with tieWaitForNote

[Kieren MacMillan]

Hello,

Any help making the snippet below work "as expected" (i.e., the ties wait for 
the chord, which is cross-staff) would be appreciated!

Thanks,
Kieren.
_______________________

\\version "2.17"
\language "english"

\layout {
  \context {
    \PianoStaff
    \consists #Span_stem_engraver
    tieWaitForNote = ##t
  }
}

upper_notes = \relative d'' {
  <<
    { \voiceOne r8 <d e>8[-. a'-. e']-. a2-. }
    \new Voice { \voiceTwo s8 \crossStaff { <b,,, c e fs b>2.. } }
  >>
}

lower_notes = \relative d, {
  \clef bass
  \tuplet 7/4 { \stemUp d32[ ~ a' ~ fs' ~ \change Staff = "piano_upper" 
\voiceTwo <b c> ~ e ~ fs ~ b] ~ }
      \change Staff = "piano_lower" \oneVoice <d,,, a' fs'>2..
}

\score {
  \new PianoStaff <<
    \new Staff = "piano_upper" \upper_notes
    \new Staff = "piano_lower" \lower_notes
  >>
}