pl/sql parameter is null sometimes

i have this sp

PROCEDURE MostrarVisitaDetalle
          (k_fecha_reg in varchar2,
           k_visnrodoc in varchar2,
.
.
.
             FROM xx.ACME vs
            where vs.visfecreg >= to_date(k_fecha_reg, 'DD/MM/YYYY')
              and vs.visfecreg < to_date(k_fecha_reg, 'DD/MM/YYYY') + 1
              and vs.visnrodoc = k_visnrodoc


As I should modify the code in the where, since the parameter is sometimes NULL