stripe->cards->listCards returns null

[gsalmon]

$stripe = new Stripe($stripeAPIKey);
$cards = $stripe->cards->listCards($stripeCustID);

$cards is empty when making a request for a valid customer ID.

$stripeAPIKey is valid, $stripeCustID is valid, stripe dashboard shows several cards for that customer ID.

Further, the stripe logs do not show any requests made when this is run.

[gsalmon]

I was not accessing the getData() method.