I have a fatal error when copying a user to another group.

Steps to reproduce:
1- I create a new user X in "Administrator users" group.
2- I click copy and then I select the "Guest accounts" group.
3- I have a page with a fatal error.

In the apache logs I find the following error message:

PHP Catchable fatal error: Argument 1 passed to eZUserType::updateUserDraft() must be an instance of eZUser, null given, called in /var/www/mysite/ezpublish_legacy/kernel/classes/datatypes/ezuser/ezusertype.php on line 365 and defined in /var/www/mysite/ezpublish_legacy/kernel/classes/datatypes/ezuser/ezusertype.php on line 312, referer: http://admin.mysite.local/content/browse/5