manual repeat mark syntax question

[Paul Scott]

What is wrong with the following code that gives:

programming error: Trying to interpret a non-markup object: (unquote 
voltaOpt)


If it isn't clear, I'm trying to create a 3rd optional ending.

\version "2.25.16"

voltaOpt = \markup{Optional}
\fixed c' {
  a1
  \repeat segno 2 {
    g1
    \volta 2
    \volta 1 {
      \repeat volta 2 { a1 b1 }
      \alternative{ c'1 d' } \bar "||"
      \set Score.repeatCommands = #'((volta ,voltaOpt))
      e1
      \set Score.repeatCommands = #'((volta #f))
    }
  }
}


TIA,

Paul