How to Successfully Add and Interface Rate Plans on Sales Order Lines to Subscription Management?

I am encountering an issue when trying to add a rate plan to a sales order line. The system returns the following error:
“Cannot get property 'UnitCode' on null object.”

• I have already set up the rate plan in pricing.
• When I add charges to the same item through the price list, the charges appear correctly on the order, but the rate plan does not.
• Both “Rate Usage with events” and “Integrate to Subscription Management” opt-ins are enabled.
• We are able to interface sales orders without rate plans to Subscription Management without problems.

Question:
How can we correctly create sales orders with a rate plan and interface them to Subscription Management without encountering errors? Are there additional setups or steps required to ensure proper processing of orders containing rate plans?