C# IOException SerialPort.Write() -- The parameter is incorrect

Question

I am trying to write on a virtual serial port. This is my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System;
using System.IO.Ports;
using System.Threading;

namespace MyWPFApp
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
/// 

public partial class MainWindow : Window
{
    static private SerialPort MyPort;

    public MainWindow()
    {
        InitializeComponent();

        MyPort = new SerialPort("COM4");
        OpenMyPort();
    }

    private void btn_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            if (!MyPort.IsOpen)
                MyPort.Open();
        }
        catch (Exception ex)
        {
            Console.WriteLine("Error opening my port: {0}", ex.Message);
        }
        Console.WriteLine(MyPort.IsOpen);
        try
        {
            MyPort.Write("Hello");
        }
        catch (Exception ex)
        {
            Console.WriteLine("Error writing:", ex.Message);
        }

        MyPort.Close();

        Console.WriteLine(MyPort.IsOpen);
    }
}
}

This is my Output:

True
A first chance exception of type 'System.IO.IOException' occurred in System.dll
Additional information: The parameter is incorrect.

The thread 0x195c has exited with code 259 (0x103).
Error writting:
False
The thread 0x19d4 has exited with code 259 (0x103).
The thread 0x1b18 has exited with code 259 (0x103).
The program '[3080] MyWPFApp.vshost.exe' has exited with code 0 (0x0).

Why I got this error?The port is correctly opened and closed and the parameters are correct too. Does someone knows where is my error?