Why is the Invoice Create REST API Returning 500?

When I try to create an invoice using the REST api I get 500 Internal Server Error both using curl and Postman. I use the same credentials to get locations and other resources so that doesn't seem to be the issue. 500 is about the most useless error a developer can receive. Any ideas?

POST

https://.../fscmRestApi/resources/11.13.18.05/invoices

Basic Auth with Username and Password

Content-Type: application/vnd.oracle.adf.resourceitem+json

Body

{    "InvoiceNumber": "MyTest",    "InvoiceCurrency": "USD",    "BusinessUnit": "US BU",    "Supplier": "My Supplier",    "InvoiceType":  "Standard",    "InvoiceAmount":  1200}