HELP!!! blank value on characterist

Hi, experts

I need a big help, I'm having a problem with data values, I have a field wich contain a transaction code, on transfer rule I have

  IF temp_comm_estr-/BIC/ZCTTRANSA = SPACE.
    RESULT = temp_comm_estr-/BIC/ZCTTRANSA.
  ELSE.
    CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT'
      EXPORTING
        INPUT  = temp_comm_estr-/BIC/ZCTTRANSA
      IMPORTING
        OUTPUT = RESULT.
  ENDIF.

But when value is equal to space and we simulate the update value is blank, if I put a value 9999 it's showed. How can I do if value is blank, show value equal 0?

Regards

Tokio