Windchill REST API Response: "Changing the subject for an Affected Product is not allowed"

I am trying to write an http POST request to create a Windchill Nonconformance (which is an object that is part of an add-on Windchill Module for Windchill Quality Management).  I am using the OData servlet.  In response to my post, Windchill rest services send back this response:

{
"error": {
"code": null,
"message": "Changing the subject for an Affected Product is not allowed."
}
}

Does anyone know that this message means?

My html body is this:

{
"Name": "NC1701B",
"Description" : "upright",
"Context@odata.bind" : "Containers('OR:wt.inf.library.WTLibrary:4148327')",
"AffectedObjects": [
{
"ID": "OR:wt.part.WTPart:152929",
"Quantity": 25,
"UnitType": {
"Value": "EACH",
"Display": "Each"
}
}]
}