Stem in double division

[Niels]

Dear users,

For a pianoscore I want to use three voices for the lower staff.
First I divide into two parts, then the upper voice again in two.
However, I am getting a warning"

ly:10:26: warning: adding note head to incompatible stem (type = 1/4)
    {\stemDown <c,,, c'>4~
                          q8 r r4}

and Lilypond compiles without a stem for te lower part (see attachment).
I decreased the stem-length of the upper voice so that there would be no 
interfering with the stem of the lower voice.
More, for the lower voice I applied \stemDown.
I have no clue how to revolve this. Could anyone help?

Regards, Niels

My code (for the lower staff):

\version "2.18.2"
music = \relative c {
  \clef bass
  <<
    {r4 <<{d'4. c8}
          \\
          {\once \override Stem.length = #'4 bes2}
        >>}
    \\
    {\stemDown <c,,, c'>4~q8 r r4}
  >>
}
\score {
  \music
  \layout{}
}

adding note head to incompatible stem.pdf
Description: Adobe PDF document