Problem with temporary polyphony

[Peter Toye]

Still a newbie here, I'm afraid. I'm trying to add a temporary extra voice to what is normally two-part polyphony using the << \\ >> notation. And it gives an error:

D:/Peter/Music/Lilypond/Purcell/VoiceBug.ly:26:7: warning: adding note head to incompatible stem (type = 1/4)
<< { 
d1. } \\ {c2 b1 } >> | 
D:/Peter/Music/Lilypond/Purcell/VoiceBug.ly:26:7: warning: maybe input should specify polyphonic voices
<< { 
d1. } \\ {c2 b1 } >> | 

I find this a bit confusing: I thought I was specifying polyphonic voices, and the offending note head is in the voice which isn't being used polyphonically. Obviously I'm doing something a bit wrong, but can't see what it is. Any kind soul able to help me?

The score is below.

Regards,

Peter
mailto:address@hidden
www.ptoye.com

\version "2.16.2"
\language "english"
global = {
\key c \minor
\numericTimeSignature
\time 3/2
}
rightOne = \relative c''' {
\global
g4 af g f ef d | 
}
rightTwo = \relative c'' {
\global
<< { d1. } \\ {c2 b1 } >> | 
}
left = \relative c {
\global
c1.
}
pianoPart = \new PianoStaff \with {
instrumentName = "Piano"
} <<
\new Staff = "right" 
<< \rightOne \\ \rightTwo >> 
\new Staff = "left" 
{ \clef bass \left }
>>
\score {
<<
\pianoPart
>>
}