TRPC error handling (on client) is confusing

const onSubmit = async (data: FormData) => {
        createThing.mutateAsync(
            {
                slug: data.slug,
                name: data.name,
            },
            {
                onSuccess: async (res) => {
                    router.push(`/${res.slug}`);
                },
                onError: (e) => {
                    const err = e.data;
                    if (err && err.code === "CONFLICT") {
                        setError(e.message);
                    }
                },
            },
        );
    };