When trying to initialize an object of the SerialPort class, it gives an error, what should I do?

Question

C++ / C#

I'm writing a plugin in Unity3D.
I want to receive data from a sensor that is connected via USB to a PC (COM3).

SerialPort port = new SerialPort();
            port.PortName = "COM3";            // assign the port name 
            port.BaudRate = 9600;               // Baudrate = 9600bps
            port.Parity = Parity.None;        // Parity bits = none  
            port.DataBits = 8;                  // No of Data bits = 8
            port.StopBits = StopBits.One;
            port.Open();
            port.Write("gettemp");
            Reference.Tell(args.sender.networkPlayer, $"Temp: {port.ReadLine()}");

But when trying to initialize an object of the SerialPort class, it gives the following error:

Invalid IL code in System.IO.Ports.SerialPort:.ctor() : Method body is Empty.

How to solve this problem?

PS Unity 5.6, .NET 4.0