Error 10327: Can't determine definition of operator ""+""

i've got the error for the operator ""-"" in the following code. I'm not sure about accessing the indivdual bits of each unsigned in the array.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use Ieee.numeric_std.all;

use work.genetica_type.all;

entity binario_fitness is

Port (

clk : in std_logic;

individuos: in genetica;

adaptacao: out fitness;

somafitness: out unsigned (7 downto 0)

);

end binario_fitness;

architecture Behavioral of binario_fitness is

begin

process (clk)

begin

If (clk 'event and clk = '1') then

for x in 0 to 49 loop

adaptacao(x) <= individuos(x)(0)-individuos(x)(1) +individuos(x)(2)- individuos(x)(3)+individuos(x)(4)- individuos(x)(5)+individuos(x)(6)-individuos(x)(7);

somafitness<=(others=>'0');

end loop;

end if ;

end process;

end Behavioral;

which includes the genetica_type in another file:

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_TEXTIO.ALL;

use ieee.numeric_std.all;

package genetica_type is

type genetica is array(0 to 49) of unsigned(7 downto 0);

type fitness is array(0 to 49) of unsigned (2 downto 0);

end package genetica_type;