[Koha] disallow hold requests to be placed on items that are not checked out

Hi, I have a question to setup "disallow hold requests to be placed on items that are not checked out". I find a preference named "AllowOnShelfHolds" and it works. If the item is not checked out, I will unable to place a hold on it. BUT if the record has more than one items, I will ABLE to place a hold on it. For example, there is a record with three items, one of them are checkout but two are not. I will ABLE to place a hold on it. Is it correct behavior or is there any setting I missed? Thanks, Justin