Relational operators

Hi,

I don't understand why the following code doesn't work, can you enlighten me ?

freq_authorization : process(clk)
begin
    if rising_edge(clk) then
        
        if ((unsigned(freq) >= "00001100" ) and (unsigned(freq) <= "00111101" )) then --compare if freq entry is between 12 and 59
                
                allowed_freq <= '1';
            else 
                allowed_freq <= '0';
        end if;
    end if;
end process freq_authorization;

If it can help, modelsim is returning# ** Warning: NUMERIC_STD.">=": metavalue detected, returning FALSE when I run.

Thanks for your help.

EDIT : freq is in std_logic_vector(7 downto 0);