diff --git a/app/PaymentDrivers/BaseDriver.php b/app/PaymentDrivers/BaseDriver.php
index dd84001f872e..8d7a022ebb81 100644
--- a/app/PaymentDrivers/BaseDriver.php
+++ b/app/PaymentDrivers/BaseDriver.php
@@ -433,7 +433,7 @@ class BaseDriver extends AbstractPaymentDriver
                     $invoice_item->tax_name2 = $fees_and_limits->fee_tax_name2;
                     $invoice_item->tax_rate3 = $fees_and_limits->fee_tax_rate3;
                     $invoice_item->tax_name3 = $fees_and_limits->fee_tax_name3;
-                    $invoice_item->tax_id = \App\Models\Product::PRODUCT_TYPE_OVERRIDE_TAX;
+                    $invoice_item->tax_id = (string)\App\Models\Product::PRODUCT_TYPE_OVERRIDE_TAX;
                 }
 
             $invoice->line_items = $invoice_items;
diff --git a/app/Services/Invoice/AddGatewayFee.php b/app/Services/Invoice/AddGatewayFee.php
index 73e7064fe265..fc1365e92337 100644
--- a/app/Services/Invoice/AddGatewayFee.php
+++ b/app/Services/Invoice/AddGatewayFee.php
@@ -81,7 +81,7 @@ class AddGatewayFee extends AbstractService
             $invoice_item->tax_name2 = $fees_and_limits->fee_tax_name2;
             $invoice_item->tax_rate3 = $fees_and_limits->fee_tax_rate3;
             $invoice_item->tax_name3 = $fees_and_limits->fee_tax_name3;
-            $invoice_item->tax_id = Product::PRODUCT_TYPE_OVERRIDE_TAX;
+            $invoice_item->tax_id = (string)Product::PRODUCT_TYPE_OVERRIDE_TAX;
         }
 
         $invoice_items = (array) $this->invoice->line_items;