DataGridViewCombobox value is not valid

hi i was retrieved the data from database and ataching to the gridview but it shows DataGridViewCombobox value is not valid. how to handle this....

This is my code.....

 private void OperationsonGridview_Load(object sender, EventArgs e)
        {

            DataGridViewCheckBoxColumn chk = new DataGridViewCheckBoxColumn();

            chk.HeaderText = "Select";

            chk.Name = "chk";

            chk.AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;

            chk.FlatStyle = FlatStyle.Standard;

            chk.ThreeState = false;

            //Creaeting ComboBox Column for DataGridView:


            DataGridViewComboBoxColumn combo = new DataGridViewComboBoxColumn();


            combo.HeaderText = "gender";

            combo.FlatStyle = FlatStyle.Standard;

            //Adding Item to ComBoxColumn:
           
            combo.Items.Add("male");

            combo.Items.Add("female");




            //Adding button Column for DataGridView:


            DataGridViewButtonColumn button = new DataGridViewButtonColumn();

            button.HeaderText = "Delete";

            button.FlatStyle = FlatStyle.Popup;

            button.DefaultCellStyle.BackColor = Color.Red;

            button.DefaultCellStyle.ForeColor = Color.White;

            //Adding All Column to DataGridView:

            dataGridView1.Columns.Insert(0, chk);

            dataGridView1.Columns.Add("", "Name");

            dataGridView1.Columns.Add("", "Qualification");

            dataGridView1.Columns.Insert(3, combo);

            dataGridView1.Columns.Insert(4, button);

            dataGridView1.Columns.Add("", "ID");

            //Hide ID Column:

            dataGridView1.Columns[5].Visible = false;

            //Assign Width For Columns:

            dataGridView1.Columns[0].Width = 70;
            dataGridView1.Columns[1].Width = 205;
            dataGridView1.Columns[2].Width = 120;
            dataGridView1.Columns[3].Width = 190;
            dataGridView1.Columns[4].Width = 90;

            //Color and Font Design For DataGridView:

            dataGridView1.DefaultCellStyle.Font = new Font("Calibri", 10.25f, FontStyle.Regular);

            dataGridView1.ColumnHeadersDefaultCellStyle.Font = new Font("Calibri", 11, FontStyle.Regular);

            dataGridView1.ColumnHeadersDefaultCellStyle.BackColor = Color.BurlyWood;

            dataGridView1.EnableHeadersVisualStyles = false;

            dataGridView1.RowHeadersVisible = false;


            dataGridView1.BackgroundColor = Color.White;
            cn.Open();
            string sql = "select * from REG_USER";
            SqlCommand cmd = new SqlCommand(sql, cn);
            SqlDataReader dr = cmd.ExecuteReader();
          

            dataGridView1.Columns[5].Visible = false;
            while (dr.Read())
            {

                DataGridViewRow row = (DataGridViewRow)dataGridView1.Rows[0].Clone();
                row.Cells[1].Value = dr["name"].ToString();
                row.Cells[2].Value = dr["qualification"].ToString();
              //here showing the error because row.Cells[3] have a combobox
                row.Cells[3].Value = dr["gender"].ToString();

                row.Cells[4].Value = "delete";
                row.Cells[5].Value = dr["id"].ToString();
                dataGridView1.Columns[5].Visible = false;
                dataGridView1.Rows.Add(row);
            }
            cn.Close();
         
        }


thank you...