datagridview

Question

I am getting an error:object reference not set to an instance of an object
ma code:
 for (int i = 0; i < dgPreveleges.RowCount - 1; i++)
            {
                if((string)dgPreveleges.Rows[i].Cells["dgColSelectAll"].Value.ToString () == "false")
                {
                    MessageBox.Show("hi");
                }
            }
plz help me to rectify this error