Error 10500

Library IEEE;

USE IEEE.NUMERIC_STD.ALL;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY mux4 IS

PORT(

mux_in_a:IN UNSIGNED(3 downto 0);

mux_in_b :IN UNSIGNED(3 downto 0);

mux_sel :IN STD_LOGIC;

mux_out :OUT UNSIGNED(3 downto 0);

);

END mux4;

ARCHITECTURE behavior OF mux4 IS

begin

if mux_sel='0' then mux_out <= mux_in_a

elsif mux_sel='1' then mux_out <= mux_in_b

end if;

end architecture behavior;

when i run this code i get the error

Error (10500): VHDL syntax error at mux4.vhd(12) near text ")"; expecting an identifier, or "constant", or "file", or "signal", or "variable"