typed/racket/gui : possible bug in slider%.get-value

The following code produces a contract error; I think TR is making
assumptions that it probably shouldn’t be.

Please could someone investigate.

Thanks,

Tim

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
#lang typed/racket/gui
(define f (new frame% [label "Frame"]))
(define sldr
   (new slider%
        [parent f]
        [max-value 0]
        [min-value -1]
        [init-value 0]
        [callback (λ (sldr cntrl-evnt)
                    (displayln (get-v)))]
        [label (format "It's false if negative")]))

(define (get-v) : (Option Index)
  (define v (send sldr get-value))
  (printf "~s ~s ~s~%" v (negative? v) (if (negative? v) #f v))
  (if (negative? v)
      #f
      (cast v Index)))

(send f show #t)
(send sldr set-value -1)
(get-v)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Runtime error is:
----------------------------------------------------------------------
broke its own contract
  promised: Index
  produced: -1
  in: Index
  contract from: cast
  blaming: cast
   (assuming the contract is correct)
  at: unsaved-editor:18.6
----------------------------------------------------------------------

-- 
Tim Brown CEng MBCS <[email protected]>
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