cannot find start of piano pedal: `Sustain'

[Jim Cline]

I have a score where the sustain starts in the voice in the bass clef, and 
ends after the voice crosses into the treble clef.  It compiles but with a 
warning:  cannot find start of piano pedal: `Sustain', with respect to the
sustainOff.  The warning goes away if I comment out midi{}.  However it 
would be nice to understand what is giving rise to the problem.  I tried 
keeping the sustainOff in the bass clef, despite the voice crossing, but 
this did not help.

Here is the measure in question:

\version "2.20.0"

upper = \relative c'' {
  \clef treble
  r2. \hideStaffSwitch\change Staff = "lower" r4*1/2 s8 \sustainOff |
}

lower = \relative c {
  \clef bass
  b8 ees, aes c, b \sustainOn ees' \showStaffSwitch\change Staff = "upper" 
aes ees' |
}

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

regards, Jim