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...