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.