Std_logic_vector With Z Character

hello all

i get this message error when i tried to to do this example can any one help me plz

Error (10315): VHDL syntax error at quad_buff.vhd(15): object with std_logic type cannot contain character 'z'

library ieee;
use ieee.std_logic_1164.all;
entity quad_tri is
port(
 a: in std_logic_vector(3 downto 0);
 g: in std_logic;
 y: out STD_LOGIC_VECTOR(3 downto 0));
end quad_tri;
architecture quad_buff of quad_tri is
begin
with g select
y <= a when '0',
  "zzzz" when others;
end quad_buff;