std.textio.read strange behaviour?!

I'm reading a text file containing values into a vhdl variable which I
ranged to [0..255].
If the values in the file are outside this range I expected the
simulator to complain,
but it happily reads in "any" integer into my byte, and displays it as
you can try below.
What am I missing? Shouldn't the assignment done in the read procedure
fail?
(tested with both modelsim and aldec - same behaviour)

Name this to a file called t.vhd (including the first comment line):

-- 1 23 456 7890

entity t is
end entity t;

use std.textio.all;

architecture t of t is
begin
  t : process is
    file my_file       : text;
    variable ok        : file_open_status;
    variable my_line   : line;
    variable to_stdout : line;
    variable comment   : string(1 to 2);
    variable good      : boolean := true;
    subtype u8 is integer range 0 to 255;
    variable byte, b2  : u8;
  begin
    file_open(ok, my_file, "t.vhd");
    while (not endfile(my_file)) loop
      readline(my_file, my_line);
      exit when my_line'length = 0;     -- only parse first non empty
lines
      read(my_line, comment);           -- throw comment
      while good loop
        std.textio.read(my_line, byte, good);
        if good then
          -- look at the output, even values larger than 255 are
printed!
          write(to_stdout, integer'image(byte)); writeline(output,
to_stdout);
          b2:= byte;                    -- this assignment works!?
--          b2:= byte+0;                  -- this fails as it should
        end if;
      end loop;
    end loop;
    file_close(my_file);
    wait;
  end process t;
end architecture t;

Regards -- Pontus