VHDL Case statement error

i create a function that generate a random number. That number represent an year. I need to find if that year is a leap-year. The range of generated numbers is between 2000 and 2017, so i think that i could use an case instead of applying a formula to find the leap years.

architecture arh_afisare of afisare is      
signal year: integer;
signal leap_year: integer;
begin
process
begin
    year <= random_gen(2000, 2017); 
    case year is
        when '2000' => leap_year <='1';
        when '2004' => leap_year <='1';
        when '2008' => leap_year <='1';
        when '2012' => leap_year <='1';
        when '2016' => leap_year <='1';
        when others => leap_year <='0';
    end case;
wait for 100 ns;
end process;
end architecture;

I get this error on every line with 'when' expect the last one.

Error: COMP96_0019: Afisare.vhd : (28, 9): Keyword 'others' expected.