Goofy error message on missing unquote

[jbclements]

Here's a simple program that's missing an unquote:

#lang racket

(require nanopass/base)

(define-language R1a
  (terminals
   (symbol (s)))
  (Ilos (l)
       (null)
       (cons s ... l)))

(define-pass all-to-a : R1a (l) -> R1a ()
  (Ilos : Ilos (l) -> Ilos ()
        [(null) `(null)]
        [(cons ,s ,l)
         (define new-syms '(a a a a))
         (define rest (Ilos l))
         `(cons new-syms ... ,rest)]))

(define-parser parse-R1a R1a)
(all-to-a (parse-R1a '(cons z (cons b (cons x (null))))))

This program produces the error

unsaved editor:13:22: all-to-a: unrecognized nonterminal in language R1a at: Ilos in: all-to-a
#(206 4)

... with a pointer to the first line of the transform. This only happens with a ...ed pattern. It took me quite a while to find this... I'm kind of baffled as to the chain that leads to this error message.