"Input is valid" means input is array, array is in correct length and the values in the array is valid. It does not mean there are no extra properties.

If your code would break because of extra properties unrelated to everything else, maybe you need to use better practices ;-)

Are you using for ... in? The prop is not inside the array, it's added to it, the items inside the array are all numbers.
I guess it's there to teach you not to use it with arrays.