[plt-scheme] Baby steps in Typed Scheme

I'm trying to figure out how to implement the earliest parts of  HtDP
in Typed Scheme, just to understand it for myself.

So, we write something like

;; A list-of-number is:
;; empty, or
;; (cons number list-of-number)

So, I tried this parameterized version in Typed Scheme

#lang typed-scheme
(require scheme/list)

(define-type-alias (ListOf X) (U Empty (Cons X)))
(define-struct: Empty ())
(define-struct: (X) Cons ((first : X) (rest : (ListOf X))))

(: sum ((ListOf Number) -> Number))
(define (sum alon)
  (cond
    [(Empty? alon) 0]
    [(Cons? alon) (+ (Cons-first alon)
                     (sum (Cons-rest alon)))]))

(make-Cons 5 (make-Cons 3 (make-Cons 2 (make-Empty))))

I get an error on the cond saying that it expected a Number, but got a
Void. (I'm assuming it doesn't think I'm exhausting my type
possibilities.)

When I replace (Cons? alon) with else, it runs, but when I try to take
the sum of the example, it says

typecheck: Wrong function argument type, expected (U Empty (Cons
Number)), got (Cons Integer) for argument 1 in: (make-Cons 5
(make-Cons 3 (make-Cons 2 (make-Empty))))

Can anybody shed some light?
Todd