Slur into alternatives

[lyuser . thegrue]

Hello,

Being quite a rookie in music notation and lilypond in general, I have a
problem:

\version "2.18.2"

{ 
  % I want a slur from the b to the c in 1) and to the a in 2)
  \repeat volta 2 { b'1 ( }
  \alternative 
  {
    { c''1 ) }
    { a'1  ) }
  }
}

This gives me a warning:

slur_to_next_alternative.ly:9:12: warning: cannot end slur
    { a'1  
           ) }

What I would like to have is a slur - indication in 2) like I have it in the
sheet that I'm trying to set in lilypond.

Oh, and the final result will be part of a choir rehearsal - maybe that's
important for the solution?

Thanks for your help,
-- 
Markus