No deserializer for CartItemMultimedia - SOAP Checkout API Errors

Error:
[15-Jun-2011 10:08:36] PHP Fatal error: Uncaught SoapFault exception: [soapenv:Server.userException] org.xml.sax.SAXException: No deserializer for {http:// objects.checkoutapi.ultracart.bpsinfo.com}CartItemMultimedia

Commonly caused by this code:

$cartItems = $client->getItems($merchantId, $itemIds);
$response = $client->updateItems($merchantId, $cart, $cartItems);

Issue:
getItems() returns back Item objects.
updateItems() wants CartItem objects.

Item and CartItem are very different objects, and SOAP is very particular. With a typeless language like PHP, the difference isn't detectable form a cursory glance. With a language like Java, a compile-time error would result.

Please see: CartItem and Item to understand the why UltraCart provides two different objects.