Quickpay API returns "BadRequest" on Refund when using RefundPaymentRequestParams with default vat_rate

[aksaniandriy]

In RefundPaymentRequestParams class, vat_rate property type is nullable int.
However, when vat_rate is not set in the instance of RefundPaymentRequestParams, PaymentService.RefundPayment method returns "BadRequest" with the following body:
{
"message": "Validation error",
"errors": {
"vat_rate": [
"is empty"
]
},
"error_code": null
}

My assumption is that vat_rate was made nullable, but RestSharp json serializer adds it to the request body event if it is null.