Order fulfillment 406 response

[dherbs2]

Hi,
We haven't been using this for a while, but now when we're using the order fulfillment method we're getting 406. It used to work just fine before.

PHP Fatal error: Uncaught PHPShopify\Exception\CurlException: Request failed with HTTP Code 406. in /libs/vendor/phpclassic/php-shopify/lib/ShopifyResource.php:544

$data = array(
"fulfillment" => array(
"tracking_company" => $tracking_company,
"tracking_number" => $shipping_tracking_number,
"notify_customer" => false
)
);

Array
(
[fulfillment] => Array
(
[tracking_company] => USPS
[tracking_number] => 943461690148539231****
[notify_customer] =>
)
)
$shopify_id = 5536948387889;
$result = $shopify->Order($shopify_id)->Fulfillment->post($data);

The other methods we're using work just fine.

Please advise!
Thanks!