[racket] Case with strings: normal behavior?

Hi,

Is this a bug or a not quite intuitive normal behavior?
> (define (foo s)
    (case s
      [("a") 'a]
      [else 'none]))
> (foo "a")
'a
> (foo (string-append "a" ""))
'none

in 5.3.1.5--2012-11-08(5589bcb/a) [3m].

Laurent
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