sum of datagridview values

Question

I have form with datagridview with columns id,name,mark .and a textbox total.Iwant to find the  sum of the mark when we entered the values into the datagridview.

int sum=0;

 private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)

        {

                        

                for (int i = 0; i < dataGridView1.Rows.Count - 1; i++)

                {

                sum += Convert.ToInt32(dataGridView1.Rows[i].Cells["mark"].Value.ToString());

                textBox1.Text = Convert.ToString(sum);

                

            }

        }

but the "Object reference not set to an instance of an object" error occurs, plz help;