TYPO3 Core

having a file fo.php in 2 folders and trying to copy one of them into the other folder leads to a fatal error

Fatal error: Call to a member function getIdentifier() on a non-object in master\typo3\sysext\core\Classes\Utility\File\ExtendedFileUtility.php on line 492

this line is

$this->writelog(2, 0, 1, 'File "%s" copied to "%s"', array($sourceFileObject->getIdentifier(), $resultObject->getIdentifier()));