Dotted half note showing as half note followed by a colon

[Patrick or Cynthia Karl]

In setting a piece where two instruments are sharing a staff, I encountered a 
dotted half note which was shown as a half note followed by a colon rather than 
by a dot.  The following illustrates the problem:

\version "2.18.0"

Aa = \relative b' {
        \oneVoice r2 r4 \voiceOne b
        \oneVoice b2. r4
}

Ab = \relative b' {
        \oneVoice r2 r4 \voiceTwo a
        \oneVoice b2. r4
}

\score {
        \new Staff { 
                        << \new Voice  { \voiceOne \Aa }
                           \new Voice  { \voiceTwo \Ab }
                        >> }
        \layout {  }
}

A similar problem is noted with the following snippet:

\new Voice <<
        { r2 r4 b' b'2. r4 } 
        { r2 r4 a' b'2. r4 }
>>

Am I doing something wrong here?  I get several warning messages, but except 
for the offending dotted half note, the pdf file looks good.