Unbound variable: \markup

[Freeman Gilmore]

The first one below works fine but the second does not work when the
dollar is replaced with \markup \vcenter \huge \bold "$".   Please
explain why?    Why is \markup bound in the first and not in the
second.    I see \markup used in many places like to the second
example that works, what is the difference?

Thank you, ƒg

 \version "2.18.2"

 dollar = \markup \vcenter \huge \bold "$"

 \layout { \context { \Voice
    \override Accidental.before-line-breaking = #(lambda (grob)
      (if (eqv? (ly:grob-property grob 'alteration) SHARP)
        (begin (ly:grob-set-property! grob 'horizontal-skylines
                 ly:grob::horizontal-skylines-from-stencil)
               (ly:grob-set-property! grob 'text dollar)
               (ly:grob-set-property! grob 'stencil
                 ly:text-interface::print))))
 } }

 { e'4 fis' bes' gis' | fis'8 gisis' fis'2 r4 }

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

 \layout { \context { \Voice
    \override Accidental.before-line-breaking = #(lambda (grob)
      (if (eqv? (ly:grob-property grob 'alteration) SHARP)
        (begin (ly:grob-set-property! grob 'horizontal-skylines
                 ly:grob::horizontal-skylines-from-stencil)
               (ly:grob-set-property! grob 'text \markup \vcenter
\huge \bold "$")
               (ly:grob-set-property! grob 'stencil
                 ly:text-interface::print))))
 } }

 { e'4 fis' bes' gis' | fis'8 gisis' fis'2 r4 }