Invalid Cursor when I want to modify itab with READ TABLE ?

Following code causes a core dump and says invalid cursor.

What should I do if I want to change gt_itab .?

REPORT ZEV_READ_TABLE.

data: begin of gt_itab occurs 0,

key like vbrk-vbeln,

name(10) type C,

amount type i,

end of gt_itab .

start-of-SELECTION.

gt_itab-key = 1.

gt_itab-name = 'erkan'.

gt_itab-amount = 10.

append gt_itab.

gt_itab-key = 2.

gt_itab-name = 'dilem'.

gt_itab-amount = 20.

append gt_itab.

LOOP AT gt_itab.

write:/ gt_itab-key,

gt_itab-name,

gt_itab-amount.

ENDLOOP.

Read table gt_itab with key Key = 1.

if sy-subrc = 0.

gt_itab-amount = 100.

modify gt_itab.

endif.

Kind Regards.

Erkan VAROL