Why is Array.isArray always returning false here?

Question

I'm trying to cast form data to a custom type, but I'm having trouble getting it to recognize an array in the custom type. I saw this method begin used in another similar problem, but it isn't working for me. Is there another way of doing this?

I need to detect the array type as using value as string splits the string character by character instead of putting the string to the first index. I can't use .push() either, because it doesn't know if object[key] is an array or not.

Simplified version of the custom type:

export type CustomType = {
    id: number;
    name: string;
    items: string[];
    [key: string]: (string | string[] | number)
}

Here is the code:

const form = document.getElementById("form") as HTMLFormElement;
const data = new FormData(form);

const object = {} as CustomType;
data.forEach((value, key)=> {
    if(Array.isArray(object[key])) {                 // <---- Always false
        const arr = value.toString().split(",");
        object[key] = arr;
    }
    else {
        object[key] = value as string;
    }
});