PHP error (Array to string conversion) on new user registration if email address is banned and " Reason shown to the banned" is empty

When a new user try to create an account and entered a banned email address if, for the corrispondent banned entry, the filed "Reason shown to the banned" ("ban_give_reason" field in "phpbb_banlist" table) is empty, the following error is returned (instead of the proper warning message):

[phpBB Debug] PHP Notice: in file [ROOT]/includes/functions_user.php on line 1560: Array to string conversion