I have an SQL union on free tables that fails when one of the subqueries is empty:

    SELECT * FROM ROOMBOOK ;
      WHERE BETWEEN(ROOM_DATE, STARTDATE, ENDDATE) .AND. ;
            !EMPTY(FOLIO_NBR) ;
      UNION ;
      SELECT * FROM RMBKHST ;
        WHERE BETWEEN(ROOM_DATE, STARTDATE, ENDDATE) .AND. ;
              !EMPTY(FOLIO_NBR) ;
      ORDER BY ROOM_DATE, ROOM_NBR ;
      INTO CURSOR C_ROOMBOOK READWRITE

If either of the tables have no matching entries, then I get an error "SELECTs are not UNION compatible. Number of fields do not match."

Can I make this work?