[racket] TR: too-narrow type in / ?

I'm dividing a Nonnegative-Real by an Index, and I want to get back a
Nonnegative-Real. The type of / contains this clause:

(-> Positive-Real Positive-Real * Nonnegative-Real)

It appears to me that the first term here should be "Nonnegative-Real",
rather than "Positive-Real"... that is, this type:

(-> Nonnegative-Real Positive-Real * Nonnegative-Real)

Am I missing something obvious here?

(Naturally, I can insert a dynamic check, it just seems unnecessary...)

Thanks!

John
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