diff --git a/class-wc-gateway-komoju.php b/class-wc-gateway-komoju.php
index 2e0c7ea..73801cf 100755
--- a/class-wc-gateway-komoju.php
+++ b/class-wc-gateway-komoju.php
@@ -197,7 +197,7 @@ public function process_payment($order_id, $payment_type = null)
             ],
             'line_items' => $line_items,
             'metadata'   => [
-                'woocommerce_order_id' => $order->get_id(),
+                'woocommerce_order_id' => strval($order->get_id()),
             ],
         ];
         $remove_nulls = function ($v) { return !is_null($v); };