[plt-scheme] Case lambda in typed scheme

While on an adventure to use typed-scheme I faced this issue:
#lang typed-scheme

(: foo (case-lambda [-> Symbol]
                    [Symbol -> Symbol]))
(define foo
  (case-lambda [() 'foo]
               [(s) s]))

I can't see what might be wrong here. Still I get in 4.1.4:
typecheck: untyped var: s in: s

Cheers,

-- 
Paulo Jorge Matos - pocmatos at gmail.com
Webpage: http://www.personal.soton.ac.uk/pocm