acciaccatura + volta bug

[CJ Bell]

I've tried the below example in 2.10.10 and 2.11.57-1 and both
generate the warning "already have a volta spanner, ending that one
prematurely" and place the grace note in it's own volta spanner.

For now, can anyone find a workaround?

Thanks!
-cj


\version "2.10.10"
%\version "2.11.57"
\score {
  \new Staff {<<
    {
      \repeat volta 2 { R1 }
      \alternative { { R1 } { R1 } }
    }
    {
      \repeat volta 2 { R1 }
      \alternative {
        { \acciaccatura c''8 c''1 }
        { R1 }
      }
    }
  >>}
}