"Rest" invisible after part combine

[Stephen Sheasby]

Hi everyone,

I am working on making some of our church’s sheet music available for overhead 
projection but I have run into a small issue that I cannot seem to get right. 
The score is an SATB hymn where the women’s and men’s voices are on separate 
staves. I have chosen partcombine instead of writing out chords because we want 
the midi to output for each voice. The problem I have run into is that my rest 
for the soprano and bass line do not print and I am wondering if this is a 
limitation of partcombine?

Code:

%       -- Common settings
global = {
        \numericTimeSignature
        \key df \major % -- Change Key as needed.
        \time 4/4
        \aikenHeads
        \partial 2
}

%       -- Soprano
sopranoNotes = \relative c' {
        \global
        r8 d8 d[ f]     \bar ".|:"                                      %  U
        f2 r4 e8 f                                              |       %  1
}

%       -- Alto
altoNotes = \relative c' {
        \global
        r8 a8 a[ d] \bar ".|:"                                  %  U
        d4 d8( c) \stemDown c4 c8 d             |       %  1
}

%       -- Tenor
tenorNotes = \relative c {
        \global
        \clef bass
        r8 f8 f[ a]                                             |       %  U
        a4 a a a8 a                                     |       %  1
}

%       -- Bass
bassNotes = \relative c {
        r8 d8 d[ d]                                     |       %  U    
        d2 d4\rest d8 d                                 |       %  1
}

Partcombine syntax:

        \score {
                <<
                        \new Staff \with { printPartCombineTexts = ##f } {
                                \partcombine \sopranoNotes  \altoNotes }

                        \new Staff \with { printPartCombineTexts = ##f } {
                                \partcombine \tenorNotes  \bassNotes }
                >>
}

Thanks in advance for your help.

Stephen