AIM PHP - Retrieving Custom Merchant Defined Fields

[js2393]

In my code I have defined a custom field:

$transaction->setCustomFields('cart_id', '1');

$response = $transaction->authorizeAndCapture();

if ($response->approved) {
  $cart_id = $response->cart_id;
  //...do stuff with $cart_id
}

 However, I get this error suggesting that "$response->cart_id" is not correct:

Notice: Undefined property: AuthorizeNetAIM_Response::$cart_id

 How do I retreive the custom fields from the $response object?

Also, in the email I receive as a merchant, only cart_id shows up (with no value) despite having defined 3 other custom fields.

========== MERCHANT DEFINED =========
0 : cart_id

 What am I doing wrong?

[js2393 · Accepted solution]

UPDATE: extremely careless mistake: I was using setCustomFields instead of setCustomField