[racket] Typed racket problem

Hi there,
I've got another problem with typed racket. 

Let us say I have an assoc list with options for a program. These
options could be of type String, Boolean, or Integer.

Now in my program I want to check certain options but I don't know how
to do without Typed Racket screaming at me.

Here a minimal example:

#lang typed/racket

(define-type Myopt (U String Boolean Integer))
(define-type OptList (Listof (Pairof Symbol Myopt)))


(: olist OptList)
(define olist (list '(dir . ".")
                    '(verbose . #t)
                    '(size . 12)))

(: optval (-> Symbol OptList Myopt))
(define (optval v ol)
  (let ([val (assoc v ol)])
    (if val 
      (cdr val)
      (error "Unexpected option encountered"))))

(: process (-> OptList Void))
(define (process ol)
  (if (<  (optval 'size ol) 0)
      (displayln "wrong value")
      (displayln "ok")))


I get an error:
Type Checker: type mismatch
  expected: Real
  given: Myopt
  in: (optval (quote size) ol)


How could I solve this?


-- 
Thanks,
Manfred