Adding Hex numbers

I want to add two hex numbers inside the process…

This is my code:

"

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

--USE IEEE.STD_LOGIC_ARITH.ALL;

USE IEEE.STD_LOGIC_UNSIGNED.ALL;

USE IEEE.NUMERIC_STD.ALL; --I using "conv_std_logic_vector" function

…

signal TX_checkSum: std_logic_vector (8 downto 0);

begin

process (clk)

begin

if reset='1' then

…

elsif clk'event and clk='1' then

TX_checkSum <= std_logic_vector(unsigned(x"55") + unsigned(x"55"));

end if;

end process; "

I get the following error:" Error (10408): VHDL error at UART.vhd(139): can't determine type of object at or near string ""01010101"" -- found 25 possible types".

What could be the problem?