How to pass a fraction as a parameter for a Scheme function

[Paolo Prete]

Hello,

I don't understand how to use a fraction as a parameter for a scheme
function. More specifically, this gives me an error:

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

proportionalNotationDur = #(define-music-function (parser location frac)
(scheme?)                          #{ \set
Score.proportionalNotationDuration = #(ly:make-moment frac) #})
{
  \proportionalNotationDur 1/16
  c' c' c' c'
}

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

How can I fix it?
Thanks!