It is impossible to add Discovery rule. After filling in required details and clicking save, following error message is displayed:
ERROR: Cannot add discovery rule
Incorrect arguments passed to function
It seems that variable $item_db_fields in the api/classes/class.citemgeneral.php function checkInput is declared in such a way as to prevent creating items without declared value_type.
Workaround is to add line
'value_type' => '0',
after the line 231 ('filter' => $filter,) in file host_discovery.php.