Updating purchase_description and purchase_rate via the API

Updating purchase_description and purchase_rate via the API

When updating an item via the API we have found we can not set the purchase_description and purchase_rate for items that do not already have values for these fields.

How can we overcome this?

We can only update these fields if the "Purchase Information" checkbox has already been checked using the UI and the product saved with some existing purchase information.

Can someone suggest a fix for this? Is there a field/flag I can update with the same API call that will allow these fields to be set?