PHP Fatal error in WooCommerce API callback

Looks like there was no order for the order key returned to the callback, but I do see the order from that time and it was successfully marked as paid.

[18-May-2022 12:40:24 UTC] PHP Fatal error:  Uncaught Error: Call to a member function get_transaction_id() on bool in /path/to/wp-content/plugins/sezzle-woocommerce-payment/woocommerce-gateway-sezzle.php:633
Stack trace:
#0 /path/to/wp-includes/class-wp-hook.php(307): WC_Gateway_Sezzlepay->sezzle_payment_callback('')
#1 /path/to/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters('', Array)
#2 /path/to/wp-includes/plugin.php(474): WP_Hook->do_action(Array)
#3 /path/to/wp-content/plugins/woocommerce/includes/class-wc-api.php(161): do_action('woocommerce_api...')
#4 /path/to/wp-includes/class-wp-hook.php(307): WC_API->handle_api_requests(Object(WP))
#5 /path/to/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters(NULL, Array)
#6 /path/to/wp-includes/plugin.php(522): WP_Hook->do_action(Array)
#7 /path/to/wp-includes/class-wp.php(396): do_action_ref_array('parse_request', Array)
#8 /path/to/wp-includes/class-wp.php(758): WP->parse_request('')
#9 /path/to/wp-includes/functions.php(1310): WP->main('')
#10 /path/to/wp-blog-header.php(16): wp()
#11 /path/to/index.php(17): require('/chroot/home/ab...')
#12 {main}
  thrown in /path/to/wp-content/plugins/sezzle-woocommerce-payment/woocommerce-gateway-sezzle.php on line 633

which is inside (abbreviated):

function sezzle_payment_callback() {
  $_REQUEST = stripslashes_deep($_REQUEST);
  $order_key = $_REQUEST['key'];
  $order_id = wc_get_order_id_by_order_key($order_key);
  $order = wc_get_order($order_id);
  $sezzle_reference_id = $order->get_transaction_id();

which is on the action:

add_action('woocommerce_api_' . strtolower(get_class($this)), array($this, 'sezzle_payment_callback'));