displaying error on datagridview's combobox column

Question

I'm trying make a new user managing panel for my winform project. Basically i want to display a combobox which lets administrative user to determine role for other users in Role column of gridview. Combobox content has just 3 items such as "Admin" , "User" , "Client" but gives me a set of error overDataError event.

All i did was right clicking on the gridview. Then click on "Edit Columns" , on the opening panel, i choose "Role" column and changed its ColumnType property to DataGridViewComboBoxColumn . Then i added name of roles into Items property's collection as one per line.

image 1: http://i.stack.imgur.com/IbHrK.png

Below you can see my code, winform design and error messages. How can i make it work without giving error messages?

image 2: http://i.stack.imgur.com/ts1ew.jpg

image 3: http://i.stack.imgur.com/gGNsy.jpg

image 4: http://i.stack.imgur.com/WBqsm.jpg

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;

namespace XXX
{
    public partial class UserManager : Form
    {
        public UserManager()
        {
            InitializeComponent();
        }

        private void UserManager_Load(object sender, EventArgs e)
        {
            panel1.BackColor = Color.FromArgb(100, 88, 55, 55);

            // TODO: This line of code loads data into the 'xXXDataSet.users' table. You can move, or remove it, as needed.
            this.usersTableAdapter.Fill(this.xXXDataSet.users);

        }

        private void button3_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {

        }

        private void dataGridView1_DataError(object sender, DataGridViewDataErrorEventArgs anError)
        {

            MessageBox.Show("Error happened " + anError.Context.ToString());

            if (anError.Context == DataGridViewDataErrorContexts.Commit)
            {
                MessageBox.Show("Commit error");
            }
            if (anError.Context == DataGridViewDataErrorContexts.CurrentCellChange)
            {
                MessageBox.Show("Cell change");
            }
            if (anError.Context == DataGridViewDataErrorContexts.Parsing)
            {
                MessageBox.Show("parsing error");
            }
            if (anError.Context == DataGridViewDataErrorContexts.LeaveControl)
            {
                MessageBox.Show("leave control error");
            }

            if ((anError.Exception) is ConstraintException)
            {
                DataGridView view = (DataGridView)sender;
                view.Rows[anError.RowIndex].ErrorText = "an error";
                view.Rows[anError.RowIndex].Cells[anError.ColumnIndex].ErrorText = "an error";

                anError.ThrowException = false;
            }
        }
    }
}