Unchecked_Union with empty variant

[Ivan Levashew <octagram@bluebottle.com>]

[Check_Unions.adb]
~~~~~~~~
procedure Check_Unions is


type Complex_Record (Kind : Integer := 0) is record
    Constant_Part : Character;
    Constant_Part2 : Character;
    case Kind is
    when 0 =>
      Variant_Part1 : Character;
    when 1 =>
      Variant_Part2 : Character;
    when 2 =>
      Variant_Part3 : Character;
    when others =>
      null;
    end case;
end record;

pragma Unchecked_Union (Complex_Record);

begin
    null;
end Check_Unions;
~~~~~~~~

It gives an error:
C:\...ramming\GEMA-Win32API\Ada_test>gnatmake -gnat05 Check_Unions.adb
gcc -c -gnat05 check_unions.adb
check_unions.adb:14:04: Unchecked_Union may not have empty component list
gnatmake: "check_unions.adb" compilation error

What's the problem?

I can't see any words "empty" or "null" here:

http://www.adaic.com/standards/05aarm/html/AA-B-3-3.html

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