C# How correctly control checkbox from another thread?

I'm trying to uncheck checkbox after timer stops. But somehow I receive an error on OnTimedEvent method in this part checkBox1.Checked = false;:

Cross-thread operation not valid: Control 'checkBox1' accessed from a thread other than the thread it was created on.

There is part of my code:

namespace WindowsFormsApplication1
{

public partial class Form1 : Form
{
    System.Timers.Timer timeris = new System.Timers.Timer();
    int test = 0;

    public Form1()
    { 
        InitializeComponent();
    }

private void checkBox1_CheckedChanged(object sender, EventArgs e)
{
    comboBox1.Enabled = (checkBox1.CheckState == CheckState.Checked);

    if (comboBox1.Enabled)
    {
        button1.Enabled = false;
    }
    else
    {
        button1.Enabled = true;
    }
}

private void button1_Click(object sender, EventArgs e)
{
    if (comboBox1.Enabled)
    {
        if (comboBox1.SelectedIndex < 0)
        {
            comboBox1.Text = "Select from list";
        }
        else
        {
            this.Hide();
            timeris.Elapsed += new ElapsedEventHandler(OnTimedEvent);
            timeris.Interval = Convert.ToInt32(test);
            timeris.Start();
        }
    }
    else
    {            
        this.Close();
    }
}

private void OnTimedEvent(object sender, EventArgs e)
{
    timeris.Stop();
    if (checkBox1.Checked == true)
    {
        checkBox1.Checked = false; // there I receive an error
    }
    this.Show();
}
}

Have you any ideas how could I solve It? If I build application and launch .exe file It looks like working successfully, but I got an error while debugging.