datagridview up-down buttons

hey guys...in my from there are two datagridviews and i fill the second datagridview with the details which was choosen from first datagridview's record..but i cant use up and down arrow buttons..i wrote such code below to use this buttons
if ((e.KeyCode == Keys.Down))
dataGridView1.DataSource = h.FillSecondGrid(dtgFirmalar.Rows[dtgFirmalar.CurrentRow.Index+1].Cells[0].Value.ToString());
but if i keep pushing the down button at tne end of the list it gives error and stops running...what should i do