why must a full measure rest (2/2) have a direction that will cause a collision?

[Kenneth Wolcott]

why must a full measure rest (2/2) have a direction that will cause a collision?

  I tried using \stemDown, but that had no effect.

warning:
========
blah.ly:34:7: warning: cannot resolve rest collision: rest direction not set
    {
      d,1\rest d,1\rest d,1\rest d,1\rest b1\rest b1\rest }
===========

My cue notes are well above the bass clef d, and well above the treble clef b.

++++++++++++++
<<
    { d,1\rest d,1\rest d,1\rest d,1\rest b1\rest b1\rest }
    \new CueVoice {
      \stemUp r4 b, cs ds'^"Tromboni" |
      e4 fs gs r |
      r4 a b cs' |
      d'4 ds' e' \cueClef "treble" \tuplet 3/2 { c''8 c'' c'' } |
      c''4 c''8 c'' c''4 c''8 c'' |
      c''4 c'' c'' c'' |
      \cueClefUnset
    }
  >>
+++++++++++++++

Thanks,
Ken Wolcott