header column in gridview

See more:

C#

my friends
i showing data in gridview

string sql = "select * "                                                                                                 + "   from table_name where t.months = '2014/05'"
                      + " order by 1 DESC";

           OracleCommand command = new OracleCommand(sql, conn);
           command.CommandType = CommandType.Text;
           OracleDataAdapter adapter = new OracleDataAdapter(command);

           DataSet dataset = new DataSet();
           adapter = new OracleDataAdapter(command);
           adapter.Fill(dataset);

           command.ExecuteNonQuery();
           GridView1.DataSource = dataset.Tables[0];
           GridView1.DataBind();

i am facing problem when i write

GridView1.Columns[1].HeaderText = "mytext";

it showing Error :

Index was out of range. Must be non-negative and less than the size of the collection.\r\nParameter name: index

can any body help me PLZ


thanks