std_logic_vector won't accept certain values

here is an example of a single process state machine:

process(clk, reset)
begin
    if reset = '1' then
        current_state <= idle;
        count         <= 0;
        
    elsif rising_edge(clk) then
    
        case current_state is
            when idle =>
                if input = '1' then
                    current_state <= doing_something;
                end if;
            
            when doing_something =>
                count <= count + 1;
                
                if count = SOME_NUMBER then
                    current_state <= finished;
                end if;
                
            when finished =>
                
                count <= 0;
                current_state <= idle;
        end case;
    end if;
end process;