[plt-scheme] quasiquote confusion with printable<%> interface

I'm trying to use the class system to model database fields. I'm
implementing the printable<%> interface, but weirdness is occurring.

Here's the Field% class:

(define Field%
  (class* object% (printable<%>)
    (init-field
     name
     [backend (new DatabaseBackend%)])

    (super-new)

    (define/public (custom-write port)
      (write `(new Field% [name ,(send backend quote-name name)]) port))

    (define/public (custom-display port)
      (display (format "Field%: ~a" name) port))))

and when I create and write a Field%, everything works as expected.

> (define f (new Field% [name "blah"]))
> f
(new Field% (name "\"blah\""))

Here's a CharField%:

(define CharField%
  (class Field%
    (init-field
     max-length)
    (inherit-field backend, name)

    (super-new)

    (define/override (custom-write port)
      (write `(new CharField%
                   [name ,(send backend quote-name name)]
                   [max-length ,max-length]) port))))

But when I try to create and write a CharField%, the unquote seems to
be ignored.

> (define cf (new CharField% [name "blah"] [max-length 20]))
> cf
(new CharField% (name ,(send backend quote-name name)) (max-length ,max-length))

What am I doing wrong?