exception of type 'System.InvalidOperationException'

System.Timers.Timer timer=new System.Timers.Timer();

private void Window_Loaded(object sender, System.Windows.RoutedEventArgs e)
		{
            this.WindowState = System.Windows.WindowState.Minimized;
            timer.Interval = 5 * 1000;
                timer.Enabled = true;
                timer.AutoReset = true;
                timer.Start();
		}

 void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            try
            {
                
                    timer.Enabled = false;
                    this.WindowState = System.Windows.WindowState.Normal;
                     .....

                
                timer.Enabled = true;
                timer.Start();

            }
            catch (Exception ex)
            {
                Logger.Log("OnTimer Error:" + ex.Message);
            }
        }