Datagridview组合框单元格接受枚举作为整数

我试图覆盖datagridview组合框列的默认行为，使其接受enum作为int。为了做到这一点，我创建了CustomComboboxCell和CustomComboboxEditingControl，如下所示：

public class CustomComboboxEditingControl : DataGridViewComboboxEditingControl
{
    public override object EditingControlFormattedValue
    {
        get
        {
            return base.EditingControlFormmatedValue;
        }
        set
        {
            if(value.GetType().IsEnum)
            {
                //convert enum to int
                base.EditingControlFormattedValue = (int)value;
            }
            else
            {
                base.EditingControlFormattedValue = value;
            }
        }
    }
}
public class CustomComboboxCell : DatagridviewComboboxCell
{
    public override Type EditType
    {
        return typeof(CustomComboboxEditingControl);
    }
}

然后在我的表单中，我创建了一个带有combobox列的datagridview来测试这个：

public enum TestEnum
{
    a = 1,
    b = 2,
    c = 3,
}
public class Test
{
    public TestEnum test {get;set;}
}
public class Form1
{
    public Form1
    {
        Datagridview dgv = new Datagridview();
        dgv.AutoGenerateColumns = false;
        DatagridviewComboboxColumn col1 = new DatagridviewComboboxColumn();
        col1.CellTemplate = new CustomComboboxCell();
        //set datasource for col1
        Dictionary<int, string> dct = new Dictionary<int, string>();
        dct.Add(1, "a");
        dct.Add(2, "b");
        dct.Add(3, "c");
        col1.Datasource = new BindingSource() {Datasource = dct};
        col1.ValueMember = "key";
        col1.DisplayMember = "value";
        col1.DataPropertyName = "test";
        dgv.Columns.Add(col1);
        dgv.Invaidate();
        this.Constrols.Add(dgv);
        //Add datasource for datagridview
        List<Test> lst = new List<Test>();
        lst.Add(new Test() {test = TestEnum.a});
        lst.Add(new Test() {test = TestEnum.b});
        lst.Add(new Test() {test = TestEnum.c});
        dgv.Datasource = new BindingList() {Datasource = lst};
    }
}

我以为一切看起来都很完美，但当我运行表单时，它仍然会给我System.ArgumentException: DataGridViewComboboxCell value is not valid.

请看一下我的代码，告诉我我错过了什么？