WinForm上のDataGridで最後の行を削除するとIndexOutOfRangeExceptionが発生

private ArrayList ar = new ArrayList();
private class c
{
    int _i;
    string _s;
    public int i{get{return _i;}}
    public string s{get{return _s;}}
    public c(int i, string s)
    {
        _i = i; _s = s;
    }
}
private void Form_Load(object sender, System.EventArgs e)
{
    ar.Add(new c(0,"a"));
    ar.Add(new c(1,"b"));
    ar.Add(new c(2,"c"));
    dataGrid1.DataSource = ar;
    DataGridTableStyle ts = new DataGridTableStyle();
    ts.MappingName = "ArrayList";
    DataGridTextBoxColumn[] cs = new DataGridTextBoxColumn[2];
    cs[0] = new DataGridTextBoxColumn();
    cs[0].MappingName = "i";
    cs[0].HeaderText = "数字";
    cs[1] = new DataGridTextBoxColumn();
    cs[1].MappingName = "s";
    cs[1].HeaderText = "文字列";
    ts.GridColumnStyles.AddRange(cs);
    dataGrid1.TableStyles.Clear();
    dataGrid1.TableStyles.Add(ts);
}
private void button1_Click(object sender, System.EventArgs e)
{
    CurrencyManager cm = (CurrencyManager)dataGrid1.BindingContext[ar];
    cm.RemoveAt(cm.Position);
    cm.Refresh();
}