problemas con el contador VHDL

Estoy tratando de hacer un contador arriba / abajo en un tablero Basys. Pero tengo algunos problemas que no puedo resolver.

Mi código:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity counter is
  Port (
    clock       : in  STD_LOGIC;
    clock_osc   : in  STD_LOGIC;
    reset       : in  STD_LOGIC;
    count_start : in  STD_LOGIC;
    count_out   : out STD_LOGIC_VECTOR (3 downto 0);
    AN1         : out STD_LOGIC;
    AN2         : out STD_LOGIC;
    AN3         : out STD_LOGIC;
    AN4         : out STD_LOGIC
  );
end counter;

architecture Behavioral of counter is
  signal count : std_logic_vector (3 downto 0);
  signal count2 : std_logic_vector (3 downto 0);

begin
  countupdown: process (clock, reset, count_start)
  begin
    if (reset = '0') then
      count <= "0000";
      count2 <= "0000";
    elsif (clock='1' and clock'event) then
      if (count_start = '0') then
        count <= count + 1;
      else 
        count2 <= count2 - 1;
      end if;
    end if;
  end process;

  count_out<= count when (clock_osc = '1') else count2;
  AN1<='0' when (clock_osc='1') else '1';
  AN2<='0' when (clock_osc='1') else '1';
  AN3<='1' when (clock_osc='1') else '0';
  AN4<='1' when (clock_osc='1') else '0' ;

end Behavioral;

Los errores:

ERROR:HDLParsers:808 - "C:/Users/sana4/Desktop/uni/Digital Design/Counter3/counter.vhd" Line 75. + can not have such operands in this context.
ERROR:HDLParsers:808 - "C:/Users/sana4/Desktop/uni/Digital Design/Counter3/counter.vhd" Line 79. - can not have such operands in this context.