This is effectively required already as you get a DB error without it.
The field is not required at the DB level (it probably should be that too) but
it has no default value so you wind up with
[nativecode=1364 ** Field 'amount' doesn't have a default value]
Note that making it required at the DB level won't change the need for this
patch & is separate. The api declares what is required at the api level
- somethings are required at the DB level but not at the api level as they are
calculated in the BAO
$params['contact_id']['api.required'] = 1;
$params['create_date']['api.default'] = 'now';
$params['frequency_interval']['api.required'] = 1;
+ $params['amount']['api.required'] = 1;
$params['start_date']['api.default'] = 'now';
$params['modified_date']['api.default'] = 'now';
}