How can I get my second tab voice to have stems!

[Patrick Horgan]

Using GNU LilyPond 2.13.45, with this source there's no stems for the
quarter notes in the bottom voice in the tab staff.  I'm attaching a png
that shows the problem.  (I turned off the footer so the png would be
small instead of being a whole page.)

\version "2.13.45"

\paper {
  oddFooterMarkup = #f
}
tagline = \markup { \center-column { "" ""   "This is my tagline" } }
upper = \relative c {
  \time 4/4
  \key c \major
  \voiceOne
  \stemUp
  < c' g' >2. <g c g'>4 |
  <c g'>4. g8 e' d c g( |
  a) d ~ d g,( a) d ~ d g,( |
 a) d ~ d g,( a) d ~ d4  |
}

lower = \relative c {
  \time 4/4
  \key c \major
  \voiceTwo
  \stemDown
  c4 e g, r | c e c e | f f f f | f f f f |
}


\score {
  <<
    \new StaffGroup = "tab with traditional" <<
      \new Staff = "guitar traditional" <<
        \clef "treble_8"
        \context Voice = "upper" \upper
        \context Voice = "lower" \lower
      >>
      \new TabStaff = "guitar tab" <<
        \tabFullNotation
        \context TabVoice = "upper" \upper
        \context TabVoice = "lower" \lower
      >>
    >>
  >>
}

lethimroll.png
Description: PNG image

lethimroll.ly
Description: Text Data