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Barrel shifter compilation in QuartusII

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I tried to compile the code presented some days ago in this newsgroup.
I use Altera QuartusII v3.0 SP2

and got the following warning:

Warning: VHDL Subtype or Type Declaration warning at
subtype or type has null range Switching left and right bound of

Was does that mean?

Apart from that I get the Info
"No valid register-to-register paths exist for clock Clk"

What does go wrong with timing calculation?


library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity barrelshifter is
port( Quantity : in unsigned(31 downto 0);
Amount : in unsigned(4 downto 0);
Reset : in std_logic;
Clk : in std_logic;
Output : out std_logic_vector(31 downto 0)
end barrelshifter;

architecture ro_lft of barrelshifter is
signal rotated : std_logic_vector(31 downto 0);
signal rotate_by : unsigned(4 downto 0);

rotate_by <= Amount;

process (Clk)
if Reset='1' then
rotated <= (others => '0');
elsif clk = '1' and clk'event then
rotated <= std_logic_vector(shift_right(quantity,to_integer(u nsigned(rotate_by))));
end if;
end process;

Output <= rotated;

end ro_lft;
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