Vertical alignment - programming error

[Andrew Bernard]

Using a grace note set that spans two staves, reduced from a vastly more 
complex real score to a more minimal example, if I use a hairpin between the 
sfz and the p indications, I get a programming error related to vertical 
alignment that I am unable to understand, or eliminate. Can folks explain this?

Andrew

— compilation log

Starting lilypond 2.19.29 [exp-k-p11-simpler.ly]...
Processing `/home/andro/work/lilypond/proportion/exp-k-p11-simpler.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers
programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers
programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers
programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers
programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
programming error: bounds of this piece aren't breakable.
continuing, cross fingers
Layout output to `/tmp/lilypond-40hIxW'...
Converting to `exp-k-p11-simpler.pdf'...
Deleting `/tmp/lilypond-40hIxW'...
Success: compilation successfully completed
Completed successfully in 0.7”.

— compilation log

— snip
\version "2.19.29"

treble = {
  \clef treble
  \time 1/4

  c''16_\sfz
  \grace {
    \bar ""
    ees'''8^>]_\sfz\> g''^> f' a' d''
    \change Staff = middle
    b fis_\p
  }
  \change Staff = top
  \bar "!"
}

bass = {
  \clef bass
  \time 1/4
  c16
}

\score {
  \new PianoStaff
  <<
    \new Staff = "top" \with {
    }
    { \treble }
    \new Staff = "middle" \with {
    }
    { \bass }
  >>
  \layout { }
}

— snip