Templates with literal ... produces an internal error

[soegaard]

The following

(Expr : Expr (e dest) -> Expr () [(let-values ,s (((,x ...) ,ce) ...) ,e) ; todo handle multiple values (let* ([x0 (map first x)] [ce (map CExpr ce x0)] [e (Expr e)]) `(let ([,x0 "undefined"] ...) (body ,ce ... ,e)))])

produces the error

../../nanopass-framework-racket/private/meta-parser.rkt:415:39: cadr: contract violation
  expected: (cons/c any/c pair?)
  given: #<syntax (quote "undefined")>

The error ought should be an external error - not an internal one.