Unable to create new Purchase Order with PO Line

{
 "notes":[],
 "order_type":"One-Time",
 "vendor":"4b05321a-3eb7-4dd4-8f1d-d6766c0e210c",
 "workflow_status":"Open",
 "po_lines":[
   {
    "cost":{"currency":"USD","list_price":"1","quantity_physical":"1"},
    "vendor_detail":{"instructions":""},
    "po_line_number":"12345-1",
    "acquisition_method":"Gift",
    "order_format":"Physical Resource",
    "title":"14 cows for America"
   }
 ]
}

{
  "errors" : [ {
    "message" : "{\"errors\":[{\"message\":\"may not be null\",\"type\":\"1\",\"code\":\"-1\",\"parameters\":[{\"key\":\"source\",\"value\":\"null\"}]}]}",
    "parameters" : [ ]
  } ]
}