Cast object with mandatory property into object with same optional property

I'm trying wrap my head around why the following doesn't work:

/* @flow */

type A = {
foo: string
}

type B = {
foo?: string
}

const a: A = { foo: 'bar' };
const b: B = (a: B);


Flow gives me:

12: const b: B = (a: B);
^ Cannot cast `a` to `B` because string [1] is incompatible with undefined [2] in property `foo`.
References:
4: foo: string ^ [1]
8: foo?: string ^ [2]


All I'm trying to do is convert an object where a property is guaranteed to exist into an object where the property might exist - shouldn't that be ok?

Try Flow link here (no idea how long this will work)

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