[racket] Optional parameters in Typed Racket?

I'm new to Typed Racket and want to define an optional parameter. I found
(->*) in the docs, but trying out the example

#lang typed/racket
>


(: append-bar (->* (String) (Positive-Integer) String))
> (define (append-bar str [how-many 1])
>   (apply string-append str (make-list how-many "bar")))


gives me this error in 6.0:

Type Checker: not a valid type: (String) in: (String)


Am I missing something obvious?

Dan
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