Error: Index was outside the bounds of the array

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;

namespace FlightSim
{
    public partial class Form1 : Form
    {
        SerialPort sp = new SerialPort();
        String[] parts = new string[4];  //4 parts bec 4 adc value
        string stx = "*0000"; //initialise stx string

        public Form1()
        {
            InitializeComponent();
        }

        private void startcommButton_Click(object sender, EventArgs e)
        {
            if (sp.IsOpen)
            {
                sp.Close();
            }
            try
            {
                sp.PortName = "COM24";
                sp.BaudRate = 115200;
                sp.Parity = System.IO.Ports.Parity.None;
                sp.DataBits = 8;
                sp.StopBits = System.IO.Ports.StopBits.One;

                sp.Open();
                sp.WriteLine("\r");
                label4.Text = "comm started";
                timer1.Enabled = true;
                
            }
            catch (System.Exception)
            {
                //Creates and returns a string representation of the current exception.
                MessageBox.Show("comm error");
            }
        }

        private void quitButton_Click(object sender, EventArgs e)
        {
            sp.Close();
            this.Close();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            //There are different methods to capture the serial data. This is one method
            stx = sp.ReadExisting();  //limit to 1 sec interval read by timer1
            parts = stx.Split('*');   //chopped it from char *
            label1.Text = parts[1];   // parts[0] is not valid, 1,2 and 3 OK
        }

        private void hScrollBar1_Scroll(object sender, ScrollEventArgs e)
        {
            //label1.Text = hScrollBar1.Value.ToString();
            //axAirGauge1.NeedleValue = hScrollBar1.Value;
        }
        private void hScrollBar2_Scroll(object sender, ScrollEventArgs e)
        {  
        }
        private void hScrollBar3_Scroll(object sender, ScrollEventArgs e)
        {  
        }      
    }
}

The error "Index was outside the bounds of the array" appeared at the line : label1.Text = parts[1];
Someone please tell me what happen..