4.20 payment method Error

Hi,
I am writing payment method for 4.20 but error.

http://esan.local/checkout/OpcCompleteRedirectionPayment
startIndex cannot be larger than length of string.
Parameter name: startIndex

I am using RemotePost method.

please help

Have you used string.substring option for create payment form?

No. Only RemotePost Method

            RemotePost post = new RemotePost
            {
                Url = $"xxxxxxx",
                FormName = "xxx",
                Method = "POST",
                NewInputForEachValue = true,
            };
             post.Add("xxx", xxxxx);
             .......
             post.Post();

thanks rajupaladiya