Why is error undefined during tests?

[IPWright83]

I'm trying to write a test around a hook using SWR. The test looks something like this:

it.only('throws an error if the response is not ok', async () => {
            globalRef.fetch.mockResolvedValueOnce({ ok: false, statusText: 'foobar' });
            const { result, rerender } = renderHook(() => useMyCustomHook());

            await act(async () => {
                await rerender();
            });

            console.log(result.current.error.message);
            expect(result.current.error).toBeDefined();
            expect(result.current.error.message).toBe('foobar');
            expect(result.current.data).toBeUndefined();
        });

What I'm finding (and I'm wondering if someone could explain why) is that error.message is undefined... that is unless I get the value within the hook itself. So:

this works

export const useMyCustomHook = () => {
    const key = "someURL";

    const result = useSWR<SomeType>(key, async () => {
        const response = await fetch(key, {
            method: 'GET',
            headers: standardHeaders(),
        });

        if (!response.ok) {
            throw Error(response.statusText);
        }

        const weather = await response.json();

        return weather.currentConditions;
    });

    return { ...result, error: result.error };
};

However if I remove the access to result.error in the return, to simply just return result, or return useSWR directly, then error is always undefined during the test.

Can anyone explain why this is as I don't understand 😢