dotted half note notehead getting filled in when merging with eighth note

[Nick Payne]

\version "2.12.2"

\relative c' {
        \mergeDifferentlyHeadedOn
        \mergeDifferentlyDottedOn
        \time 3/4
        <<
        { \voiceOne c''4 s b } 
        \new Voice { \voiceTwo \stemUp e,,,8[ \stemDown gis'] fis'[ d] ~ d[
fis] } 
        \new Voice { \voiceFour e,,2. }
        >>
}

In the fragment above, the notehead for the dotted half note in voiceFour is
being filled in so that it looks like a dotted quarter note. Should this be
happening? The nearest example I can find in the documentation (subsection
on Collision Resolution in NR 1.5.2) shows a half note being merged with an
eighth note and the notehead remaining unfilled. How can I prevent the
notehead above being filled in?

Nick