I am extending existing interfaces with additional properties, which works fine

interface BaseAnimal {
    name: string,
    age: number
}

interface Dog extends BaseAnimal {
    breed: string,
    alive: boolean
}

interface Cat extends BaseAnimal {
    origin: string,
    height: number,
    furious: boolean
}

type Animal = (Dog | Cat)
type Animals = (Dog | Cat)[]


type WithAdditionalProperties<T> = {
    properties: T;
    city: string | null;
    country: string
};

export type AdditionalAnimalProps = WithAdditionalProperties<Animals[number]>;
export type AdditionalAnimalsProps = WithAdditionalProperties<Animals[number]>[];

However, when I change the type of

type Animals = (Dog | Cat)[]

to

type Animals = (Dog | Cat)[] | undefined

this

export type AdditionalAnimalProps = WithAdditionalProperties<Animals[number]>;
export type AdditionalAnimalsProps = WithAdditionalProperties<Animals[number]>[];

is not working anymore, which is expected.

Type Animals has no matching index signature for type  number

How do I get around this? Here is a Playground