Missing type conversion yields constraint error

[senier]

package Test is

   type Length is mod 2 ** 8;
   type Elems is range 0 .. 1 with Size => 8;

   type Data is
      message
         Length : Length;
         Value  : Opaque
            with Size => 8 * Length;
      end message;

   type Message (L : Length) is
      message
         Elems : Elems;
         Data  : Opaque
            with Size => 8 * L * Elems;
      end message;

   generic
   session S with
      Initial => Init,
      Final   => Done
   is
      M : Message;
      D : Data;
      E : Elems;
   begin
      state Init
      is
      begin
         M := Message'(L     => 64,
                       Elems => E,
                       Data  => D.Value);
      transition
         goto Done
      exception
         goto Done
      end Init;

      state Done is null state;
   end S;

end Test;

gprbuild -P build
Compile
   [Ada]          rflx-rflx_builtin_types-conversions.ads
   [Ada]          rflx-test-s_allocator.adb
   [Ada]          rflx-test-s.adb
rflx-test-s.adb:27:147: error: value not in range of type "Elems" defined at rflx-test.ads:44
rflx-test-s.adb:27:147: error: static expression fails Constraint_Check
rflx-test-s.adb:28:169: error: value not in range of type "Elems" defined at rflx-test.ads:44
rflx-test-s.adb:28:169: error: static expression fails Constraint_Check
gprbuild: *** compilation phase failed
make: *** [Makefile:5: all] Error 4