I wanted to test "adding" an item to an order via the API

I used the API to fetch an order's details (GET /CustomerOrder/ID) into an object and added an item

i then called the PUT method for /CustomerOrder/ID which failed, and the HTTP response body says "Unexpected Error" and a JSON dump of my request

I don't know where I go from here? What error'd? Are there properties that I can't change?

I also tried just PUTting back the exact same data I received from the GET request but that had the same issue