Need help on how to use functions correctly

[Panic]

I am working on an implementation of the Rijndael encryption algorithm,
and in one of my behavioral architectures, I call a function from a package
I have written, that do a simple table lookup to transform a eight bit
vector
into another eight bit vector.

My problem is that I keep getting error messeges from my compiler, that
complaints about "Prefix name 'sub_byte' is not a 1 dimentional
array or a function or contains a bad suffix (index/slice/selected-name)"
when I try to call the function sub_byte (that belongs to the package
srd_lookup_pkg)
from a process in the behavioral architecture of my key_shedule-file.

I have tried to supply the function with an explicit argument, like
sub_byte(x"63"), and with a signal of the correct type (statebyte),
but nothing seems to help. So what am I missing here?

I use the compiler that comes with Symphony EDA Sonata 2.1 (Free Edition).

Sincerely
-Panic

I include some of the contents of some of the files, so you all can get
an impression on how my function call fits in to my design:

----------------------------------------------------------------------------
---------------
srd_lookup_pkg.vhd
----------------------------------------------------------------------------
---------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

package srd_lookup_pkg is
subtype statebyte is STD_LOGIC_VECTOR (7 downto 0);
subtype halfbyte is STD_LOGIC_VECTOR (3 downto 0);
end package srd_lookup_pkg;

package body srd_lookup_pkg is
function sub_byte (signal state : in statebyte ) return statebyte is
variable sub : statebyte;
begin
case state is
WHEN x"00" => sub := x"63";
[...]
WHEN x"FF" => sub := x"16";
WHEN OTHERS => sub := x"00";
end case;
return sub;
end sub_byte;
end package body srd_lookup_pkg;

----------------------------------------------------------------------------
---------------


----------------------------------------------------------------------------
---------------
key_schedule.vhd
----------------------------------------------------------------------------
---------------
library ieee;
use ieee.std_logic_1164.all;
use work.types.all;
use work.srd_lookup_pkg.all;

entity key_schedule is
port ( key : in std_logic_vector (127 downto 0); -- 128 bits nøkkel
load_key : in std_logic; -- 1 = load, 0 = norm. op.
reset : in std_logic; -- Reset
round : in integer range 1 to 10; -- Angir runde
round_key : out std_logic_vector (127 downto 0) ); -- =
ExpandedKey[round]
end entity key_schedule;

architecture behave of key_schedule is
signal srd_tmp : statebyte;
begin
load : process(load_key) is
variable expKey : exp_key;
variable RC : rcs;
begin

-- Short
for j in 4 to 43 loop
if j mod 4 = 0 then
srd_tmp <= x"A2";
expKey(j)(0) := expKey(j-4)(0) xor sub_byte(srd_tmp) xor
RC(j/4);
else
-- blah blah...
end if;
end loop;

end process load;
end architecture behave;
----------------------------------------------------------------------------
---------------


----------------------------------------------------------------------------
---------------
types.vhd
----------------------------------------------------------------------------
---------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use work.srd_lookup_pkg.all;

package types is
type rcs is array (0 to 10) of statebyte;
type col is array (3 downto 0) of std_logic_vector (7 downto 0);
type state is array (3 downto 0) of col;
type exp_key is array (43 downto 0) of col;
end package types;