getting a php error on isset() for multidimentional array

I am getting a php error on this line

if (isset($this->contents[$products_id])) {

my function:

class shoppingCart {
    public ?array $contents = null;

function get_quantity($products_id) {
      if (isset($this->contents[$products_id])) {
        return $this->contents[$products_id]['qty'];
      } else {
        return 0;
      }
}
}

The array is defined public, yet it seems to be undefined, or at least a problem when empty?

Thanks for any thoughts!

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