Check for empty field in selection screen.

Hi all,

how can I achieve this ?

Selection screen:

1. Sales  Document number -- range
2. 2.Date on Which Record Was Created – range

Validate selection screen:

Mandatory fields, user has to enter either 1 OR 2 OR both .

selection-screen begin of block b1 WITH FRAME.

select-OPTIONS: so_vbeln for vbak-vbeln ,
                                so_erdat for vbak-erdat.

SELECTION-SCREEN end of BLOCK b1.

At selection screen.
  IF so_vbeln = 0 .
    IF so_erdat = 0.
      MESSAGE 'CANNOT EMPTY'  TYPE 'E'.
    ENDIF.
  ENDIF.

I am not able to validate my selection screen.

please help.

Thanks.