Prolific USB Serial Exception when Baud rate is set

Anyone has this issue. It works ok with the FTDI.
But my production boards have prolific, so has to solve it


         private static void UsbSerialConnected(object sender, UsbSerial usbSer)
         {
             Program.DB_Print("UsbSerialConnected");

             usbSerial = usbSer;

             usbSerial.BaudRate = 9600;   <== Exception here

             usbSerial.DataReceived += usbSerial_DataReceived;

         }

UsbSerialConnected
#### Exception System.Exception - 0xffffffff (7) ####
#### Message:
#### GHI.Usb.Host.UsbSerial::NativeSetBaudRate [IP: 0000] ####
#### GHI.Usb.Host.UsbSerial::set_BaudRate [IP: 0010] ####
#### GHI.Usb.Host.Controller::OnUsbSerialConnected [IP: 000d] ####
#### GHI.Usb.Host.Controller::OnConnect [IP: 00b0] ####
#### GHI.Utilities.InternalEvent::RaiseEvent [IP: 000d] ####
#### GHI.Utilities.InternalEvent::OnEvent [IP: 003e] ####