check for entries on the selection screen

if one of the select options is obligatory with no default value then the radio buttons does not work. Can anybody please let me know why. it is the same if i dont make the field obligatory but write code in at selection-screen on block with a error message.

REPORT ZSELECT1 .

tables : ekko.

data: group_id(3) type c.

SELECTION-SCREEN BEGIN OF BLOCK MAIN WITH FRAME TITLE TEXT-001.

PARAMETERS: p_out RADIOBUTTON GROUP RADI DEFAULT 'X' USER-COMMAND

CHECK,

p_file RADIOBUTTON GROUP RADI.

SELECTION-SCREEN END OF BLOCK MAIN.

SELECTION-SCREEN BEGIN OF BLOCK output WITH FRAME TITLE TEXT-002.

SELECT-OPTIONS: so_ekgrp FOR EKKO-EKGRP obligatory MODIF ID OUT,

so_ekorg FOR EKKO-EKORG MODIF ID OUT.

SELECTION-SCREEN END OF BLOCK OUTPUT.

SELECTION-SCREEN BEGIN OF BLOCK UPLOAD WITH FRAME TITLE TEXT-003.

PARAMETERS: p_pcfile LIKE rlgrap-filename MODIF ID UPL,

p_dismod(1) TYPE c DEFAULT 'N' MODIF ID UPL.

SELECTION-SCREEN END OF BLOCK UPLOAD.

AT SELECTION-SCREEN OUTPUT.

CASE 'X'.

WHEN P_OUT. GROUP_ID = 'UPL'.

WHEN P_file. GROUP_ID = 'OUT'.

ENDCASE.

LOOP AT SCREEN.

IF SCREEN-GROUP1 = GROUP_ID.

screen-input = 0.

MODIFY SCREEN.

ENDIF.

ENDLOOP.