More trill questions

[Martin]

Hi again,
I don't understand why the example below doesn't work.
From what I can see, starting with a trill in a polyphonic passage triggers a 
"syntax error, unexpected EVENT_IDENTIFIER".
I got it to work is the trill is on the 2nd note...
I tried writing out the voices ( "\new Voice { ... }" ) but it didn't work 
either.
Thanks for your help!
Martin

\version "2.18.2"
\score {
  \new Staff {
  <<
    {
      c'2 c'
    }
    \\
    {
      \trill c2 c
    }
  >>
  }
}