Assertion error on Gauche

er.scm

(import (scheme base) (scheme write) (scheme cxr))

(cond-expand
 (chibi (import (chibi)))
 (gauche (import (gauche base)))
 (sagittarius (import (sagittarius)))
 (else (error "not supported")))


(define-syntax comp
  (er-macro-transformer
   (lambda (form rename compare)
     (display (compare (cadr form) (caddr form))) (newline)
     `(,(rename 'begin)))))

(define-syntax rename-it
  (er-macro-transformer
   (lambda (form rename compare)
     (rename (cadr form)))))

(let ((a 1))
  (comp (rename-it a) (rename-it a)))