Order beforeSave plugin throws TypeError

I’ve built a plugin that catches the order beforeSave event in order to add some extra data to the an order attribute. The problem is that when this plugin fires it causes an error:

Uncaught TypeError: Argument 1 passed to Magento\Sales\Model\OrderRepository::save() must implement interface Magento\Sales\Api\Data\OrderInterface, string given, called in /var/www/html/vendor/magento/framework/Interception/Interceptor.php on line 58 and defined in /var/www/html/vendor/magento/module-sales/Model/OrderRepository.php:156 Stack trace: #0 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Sales\Model\OrderRepository->save('sales_order', 'order', NULL, NULL, NULL, NULL, Array, NULL, NULL, Array, true, 'order', Object(Magento\Store\Model\StoreManager), Object(Magento\Sales\Model\Order\Config), Object(Magento\Catalog\Model\ProductRepository\Interceptor), Object(Magento\Catalog\Model\ResourceModel\Product\CollectionFactory), Object(Magento\Sales\Model\ResourceModel\Order\Item\CollectionFactory), Object(Magento\Catalog\Model\Product\Visibility), Object(Magento\Sales\Model\Service\InvoiceService), Object(Magento\Directory\Model\CurrencyF" 

I’ve cut down my plugin so it does absolutely nothing other than return the same object in gets at the start and I’m still getting this same issue. For example;

class OrderPlugin {  protected $  _logger; protected $  _orderApi; protected $  _orderAttributeFactory; protected $  _orderExtensionFactory;  public function __construct(\Psr\Log\LoggerInterface $  logger, Data $  helper, \MyModule\Shipstation\Model\OrderAttributeFactory $  orderAttributeFactory, OrderExtensionFactory $  orderExtensionFactory) {     $  this->_logger = $  logger;     $  this->helper = $  helper;     $  this->_orderAttributeFactory = $  orderAttributeFactory;     $  this->_orderExtensionFactory = $  orderExtensionFactory; }  public function beforeSave(\Magento\Sales\Api\OrderRepositoryInterface $  subject, \Magento\Sales\Api\Data\OrderInterface $  order){      //$  resultOrder = $  this->saveFarmisonOrderIDAttribute($  resultOrder);      return $  order;  } 

my di.xml looks like;

<type name="Magento\Sales\Api\OrderRepositoryInterface">     <plugin name="MyModule_Shipstation::orderPlugin" type="MyModule\Shipstation\Plugin\OrderPlugin" />     <plugin name="MyModule_Shipstation::deliverydatePlugin" type="MyModule\Shipstation\Plugin\DeliveryDatePlugin" /> </type> 

If I totally comment out my beforeSave method then the save works correctly as expected/