Setting Default Entity On a Custom Transaction Form

I'm creating a Custom Transaction Form to be used by a specific role. Users with this role will only be placing orders for a specific entity/customer. My aim is to make this process as simple as possible for them by defaulting as many fields as possible via workflow. I've created the workflow, it sets all of the default fields when a new sales order is created, but the order won't save. I get an "unexpected error" message. I have identified which workflow action is causing it: set field value-entity=________. No matter how much I tweak the workflow, nothing seems to