Disable radiobutton

Hi,

I have to desactivate a radio button defined like this :

SELECTION-SCREEN:BEGIN OF BLOCK B1 WITH FRAME TITLE TEXT-T10.
PARAMETERS P_MATNR TYPE MARC-MATNR .
SELECTION-SCREEN:BEGIN OF BLOCK B2 WITH FRAME.
SELECTION-SCREEN BEGIN OF LINE.
PARAMETERS  P_RB1 RADIOBUTTON GROUP GR1 USER-COMMAND CMD DEFAULT 'X'.
             SELECTION-SCREEN COMMENT (50) TEXT-010 FOR FIELD P_RB1.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN BEGIN OF LINE.
PARAMETERS   P_RB2 RADIOBUTTON GROUP GR1.
             SELECTION-SCREEN COMMENT (50) TEXT-011 FOR FIELD P_RB1.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN :  END OF BLOCK B2.
SELECTION-SCREEN :  END OF BLOCK B1.

And when I want to desactivate , the script goes well but I receive a dump

FORM DISABLE_BR USING BR TYPE CHAR5.

LOOP AT SCREEN.
IF SCREEN-NAME EQ BR.
   SCREEN-INPUT = 0.
   MODIFY SCREEN.
   EXIT.
ENDIF.
ENDLOOP.
ENDFORM.     

Help me please

Thank you