alist problem

[David Sumbler]

IR 2.19.82 shows the following amongst the settings for TrillSpanner:

bound-details (list):
'((left (text #<procedure musicglyph-markup (layout props glyph-name)>
              "scripts.trill")
        (Y . 0)
        (stencil-offset -0.5 . -1)
        (padding . 0.5)
        (attach-dir . 0))
  (left-broken (end-on-note . #t))
  (right (Y . 0)))

If I write

\override TrillSpanner.bound-details.left-broken = #'(end-on-note . #t)

Lilypond accepts it, suggesting that I am using the correct syntax.

But if I write

\override TrillSpanner.bound-details.right = #'(Y . 0)

Lilypond produces:

ERROR: Wrong type argument in position 2 (expecting association list):
(Y . 0)

I can't figure out why this is.  Can somebody please explain it to me,
and show me the correct syntax?

(The reason for doing this was to try to shorten the extent of a trill
spanner.  This may not be the correct parameter to change, which is why
I was trying different values; but even if that is true, I would still
like to know why my line produces an error.)

David