Upload control, value property, object reference not set to an instance of an object

I have an app that depends on some attachments in the data model. In the first form, my users can add and remove attachments, and in the second form, they can review the attachments they added to the request, minus the ability to add and remove. This all works fine. Regardless if attachments are added or not, the request will be forwarded to a sub-process for routing. If I have no attachments on the initial request, the workflow will show the next step of the UI in the web browser. If, however I have added attachments, I get the following error when I reach the first step in the sub-process:

Error Type: BaseException

Error Message: Unable to process control: bc3f84a6-bc06-4636-954e-2ff88171c800. Control type: Upload. Xpath: Approval.Attachments. Unable to process property: VALUE. Object reference not set to an instance of an object.

This only occurs when I add attachments to the process at the start. I believe I am updating the data model for the transactional sub-process correctly as such:

//Subprocess attachments = main process attachment

<FleetMemo.Approval.Attachments> = <FleetMemo.Attachment>;