Numbers as easy note heads with RhythmicStaff

[Gianmaria Lari]

This code 

\version "2.19.81"

\new RhythmicStaff { \easyHeadsOn a4 b8 a }

generate this score:

That's great. 

Now instead of the note name, I would like to write the numbers representing the scale-degree inside the note head. So I tried adding to the previous code the snippet I found in the documentation but it doesn't compile. This is code..

\version "2.19.81"

#(define Ez_numbers_engraver

   (make-engraver

    (acknowledgers

     ((note-head-interface engraver grob source-engraver)

      (let* ((context (ly:translator-context engraver))

     (tonic-pitch (ly:context-property context 'tonic))

     (tonic-name (ly:pitch-notename tonic-pitch))

     (grob-pitch

      (ly:event-property (event-cause grob) 'pitch))

     (grob-name (ly:pitch-notename grob-pitch))

     (delta (modulo (- grob-name tonic-name) 7))

     (note-names

      (make-vector 7 (number->string (1+ delta)))))

(ly:grob-set-property! grob 'note-names note-names))))))

\layout {

  ragged-right = ##t

  \context {

    \Voice

    \consists \Ez_numbers_engraver

  }

}

\new RhythmicStaff { a4 b8 a }

and this is the error I get

Interpreting music..document.ly:8:26: In procedure ly:pitch-notename in _expression_ (ly:pitch-notename tonic-pitch):

..document.ly:8:26: Wrong type argument in position 1 (expecting Pitch): ()

Any suggestion to fix it?

Thank you, g.