Setting dropbox "out of"/ScoreDenominator from API

I'm trying to create an assignment dropbox and a new grade item from the API, with the dropbox linked to the new grade item. Everything works in isolation, but when I add the AssociatedTool parameters to the call which creates the Grade object (to link it to the dropbox), it generates a 500 error.  Trial and error has shown that this is a result of the dropbox not having the "Out of" field set, which seems to translate to the ScoreDenominator value in the JSON block. This seems to be read only, if I try to set it the value I supply gets ignored and the result shows it as null. I can't set that parameter either in the initial POST request to create the dropbox or using a PUT request to update it. Am I missing something, or is there a better way to do this? Example input: { "CategoryId": null, "Name": "(01-2) Computing assignment 1a", "Assessment": { "ScoreDenominator": 100.000000000, "Rubrics": [] }, "CustomInstructions": { "Content": "", "Type": "Text/Html" }, "Availability": null, "GroupTypeId": null, "DueDate": "2018-04-12T16:00:00.000Z", "DisplayInCalendar": true, "NotificationEmail": null, "IsHidden": false}  Example output: { "Id": 68, "CategoryId": null, "Name": "(01-2) Computing assignment 1a", "CustomInstructions": { "Text": "", "Html": "" }, "Attachments": [], "TotalFiles": 0, "UnreadFiles": 0, "FlaggedFiles": 0, "TotalUsers": 1, "TotalUsersWithSubmissions": 0, "TotalUsersWithFeedback": 0, "Availability": null, "Assessment": { "ScoreDenominator": null, "Rubrics": [] }, "GroupTypeId": null, "DueDate": "2018-04-12T16:00:00.000Z", "DisplayInCalendar": true}