[racket] typing a function

Hello,

can someone explain me the error I get for this function, and how to fix it?

Thanks in advance.

========
#lang typed/racket

(define-type (Array a) (Rec AA (U (Vectorof a) (Vectorof AA))))

(: make-array (All (a) ((Listof Positive-Index) a -> (Array a))))

(define (make-array dims init)
  (let loop ((dims dims))
    (match dims
      ((list first)
       (make-vector first init))
      ((list-rest first rest)
       (let ((v (make-vector first)))
         (for ((i (in-range first)))
           (vector-set! v i (loop rest)))
         v)))))
========

Welcome to DrRacket, version 5.3 [3m].
Language: typed/racket [custom]; memory limit: 128 MB.
. Type Checker: Polymorphic function vector-set! could not be applied
to arguments:
Argument 1:
  Expected: (Vectorof a)
  Given:    (Vectorof Integer)
Argument 2:
  Expected: Integer
  Given:    Integer
Argument 3:
  Expected: a
  Given:    (Rec AA (U (Vectorof a) (Vectorof AA)))

Result type:     Void
Expected result: Any
 in: (vector-set! v i (loop rest))
. Type Checker: Expected (Rec AA (U (Vectorof a) (Vectorof AA))), but
got (Vectorof Integer) in: v
. Type Checker: Summary: 2 errors encountered in:
  (vector-set! v i (loop rest))
  v
>