Serial Port - problem with Write()

Hello All, 

I trying send text via serial port and I have a problem with Write function. When this function is running, application stop and I must close app from task menager.

AX2012 is running on server, I connect via RDP with ports inherit.

What is wrong?

My code:

static void Job2(Args _args)
{
System.IO.Ports.SerialPort SerialPort;

SerialPort = new System.IO.Ports.SerialPort();

SerialPort.set_PortName("COM4");
SerialPort.set_BaudRate(9600);
SerialPort.set_Parity(System.IO.Ports.Parity::None);
SerialPort.set_DataBits(8);
SerialPort.set_StopBits(System.IO.Ports.StopBits::One);
SerialPort.Open();

SerialPort.Write("something");

}