输出字符串不正确

这在Delphi 2009中运行良好:

procedure TForm1.Button1Click(Sender: TObject);
var
  Buff: UnicodeString;
  TheSize: Int64;
begin
  SetLength(Buff, 255);;
  TheSize := 1024768;
  StrFormatByteSizeW(TheSize, PWideChar(Buff), Length(Buff));
  Label1.Caption := Buff;
end;