shipment API / Order not Found, or no implicit conversion of Symbol into Integer

I am having issues with the shipment API. We use a similar process to update inventory via POST, but we are unable to get tracking to work at all.

This is the JSON I am sending:

{"shipment":{"sb_order_seq":1087,"carrier_name":"FedEx","tracking_number":"561514290845","items":{"sku":"GXF05753","quantity":1}}}

I get the response

"There was an error with your request. (no implicit conversion of Symbol into Integer)"}}