diff --git a/app/Services/InquiryService.php b/app/Services/InquiryService.php
index 4f23517..5380141 100644
--- a/app/Services/InquiryService.php
+++ b/app/Services/InquiryService.php
@@ -615,7 +615,7 @@ class InquiryService extends BaseService
 
         $options = array();
         $options['transaction_id'] = $order_inquiry['escrow_trade_no'];
-        $options['out_refund_no'] = $inquiry_refund_no;
+        $options['out_refund_no'] = (string)$inquiry_refund_no;
         $options['reason'] = $refund_reason;
         $options['amount'] = [
             'refund' => (int)$order_inquiry['payment_amount_total'] * 100,