Why are my full-measure rests engraved as two rests?

[Henry Law]

According the the Notation Reference (1.2.2) "A full-measure rest is 
printed as either a whole or breve rest, centered in the measure, 
depending on the time signature". But on my 2.18.2 installation the 
following

\version "2.18.2"

\score {
  <<
    \new Voice \with {
      \consists "Pitch_squash_engraver"
    } {
      \time 4/4
      \improvisationOn
      bes4. c8~c4 r4 |
      \compressFullBarRests
      R1*3 |
    }
  >>
}

... produces whole-measure rests represented curiously as a breve and a 
whole-note rest, as shown in 
http://www.lawshouse.org/music/minimal_example.jpg  I tried it without 
Pitch_squash_engraver, and also without improvisationOn and got the same 
results.

Sigh ... I expect it's me, but I can't see what I'm doing wrong.

--

Henry Law            Manchester, England