Tester hlasi chybu v vendor/nette/utils/src/Utils/DateTime.php

Ucim se jak pouzivat Tester, viz https://tester.nette.org/

Pri spusteni testu pres ‚php ./tests/greeting.phpt‘ funguje vse jak ma. Nicmene pri spusteni pres tester obdrzim info o chybe v nette/utils/src/Utils/DateTime.php

$ php ./vendor/nette/tester/src/tester.php ./tests/greeting.phpt
 _____ ___  ___ _____ ___  ___
|_   _/ __)( __/_   _/ __)| _ )
  |_| \___ /___) |_| \___ |_|_\  v1.7.1

Note: No php.ini is used.
PHP 5.6.31 | php-cgi | 8 threads

F

-- FAILED: tstprj/tests/greeting.phpt
   Exited with error code 255 (expected 0)
   <br />
   <b>Fatal error</b>:  Interface 'JsonSerializable' not found in <b>/usr/local/tstprj/vendor/nette/utils/src/Utils/DateTime.php</b> on line <b>15</b><br />


FAILURES! (1 test, 1 failure, 0.0 seconds)

Hlavicka te tridy vypada takhle:

namespace Nette\Utils;

use Nette;

/**
 * DateTime.
 */
class DateTime extends \DateTime implements \JsonSerializable {
...

Co je spatne? Ten interface by mel byt normalne pristupny, ne? Kdyz jej zakomentuju, tak uz se tester chova tak jak ma.