[racket] Problems with Typed Racket

I was trying Typed Racket for the first time.

when I do "Check Syntax" on the following function (which in a (untyped) Racket program works correctly) (I simplified the real function):

#lang typed/racket

(: elaborate (String String -> Any))
(define (elaborate in-path-name out-path-name)
  (let ([out-path (string->path out-path-name)])
        (call-with-output-file out-path #:exists 'replace
          (lambda(out-file)
            (list out-file)))))

the following error message is shown: "for: expected a sequence for (v r), got something else: #f"

If I omit #:exists 'replace
then the message becomes (notes that in the reference manual it is said that call-with-output file expects a path as first parameter):

<unsaved editor>:6:8: Type Checker: Polymorphic function call-with-output-file could not be applied to arguments:
Argument 1:
  Expected: String
  Given:    Path
Argument 2:
  Expected: (Output-Port -> a)
  Given:    (Any -> (List Any))

Result type:     a
Expected result: Any
 in: (call-with-output-file out-path (lambda (out-file) (list out-file)))


Am I doing something wrong?
Or Typed Racket is an experimental feature?

Thanks very much for your help!

Renzo