Error while placing order. PSI-0013:Invalid SubTotal

Hi,
I'm using nop 1.8 with PSI Gate as merchant account.
I'm getting this error for orders over $1000. It looks like the error is caused by a comma being added if the subtotal is over 1000.
I used Replace(",", "") to fix it in \Payment\Nop.Payment.PSIGate\PSIGatePaymentProcessor.cs

builder.Append("<Subtotal>" + XmlHelper.XmlEncode(paymentInfo.OrderTotal.ToString("N2", new CultureInfo("en-US", false).NumberFormat).Replace(",", "")) + "</Subtotal>");

Thanks for info