PartitionKey extracted from document doesn't match the one specified in the header c#

Question

I don't understand what is happening here. I have some code (which has been in place for ages). It looks like this:

public async Task CreateAsync(T model, string partitionKey)
{
    await _client.CreateDocumentAsync(
        UriFactory.CreateDocumentCollectionUri(
            _databaseName,
            _collectionName
        ),
        model,
        new RequestOptions
        {
            PartitionKey = new PartitionKey(partitionKey)
        });
}

I have created a new database (cosmos db) and when I tried to save a record, I got the error:

PartitionKey extracted from document doesn't match the one specified in the header

Which is odd. It was working fine before and infact, if I take the json and use it in azure, it works perfectly. Someone else had a similar issue and they stated that the solution was to upgrade my packages (specifically JSON.NET) which I have done, but I am still getting the error.

The JSON I used, looks like this:

{
    "_id" : ObjectId("5c3e516647d3c103a0ad0959"),
    "gtin" : "4548736035652",
    "modelAlias" : "KDL-32WD756",
    "modelGroup" : "WD75",
    "retailerTitle" : "Sony Bravia 32WD756BU LED HD 1080p Smart TV, 32 with Freeview HD & Cable Management System",
    "shortTitle" : "Sony WD75 32 inch Black",
    "summary" : "The Sony WD75 in Black is a smart TV, and comes with a 32 inch, full HD, LED screen.",
    "variant" : "4548736035645",
    "colour" : "Blacks",
    "resolution" : "Medium",
    "curved" : "No",
    "panelType" : "LED",
    "3D" : "No",
    "size" : "Small",
    "goodOnWall" : "No",
    "darksAndBrights" : "Medium",
    "refreshRate" : "Standard",
    "richerColours" : "No",
    "wiFi" : "Yes",
    "smart" : "Yes",
    "recordable" : "Yes",
    "latestModel" : "No",
    "soundQuality" : "Low",
    "voiceActivated" : "No",
    "inputs" : "Two",
    "pictureQuality" : "Medium",
    "brand" : "Sony",
    "energyEfficiency" : "Medium",
    "sizeBoundary" : "Upper small",
    "mountabilityFunction" : "Bad",
    "screenSize" : "31-40",
    "viewingAngle" : "Wide",
    "descriptions" : [
        {
            "criteriaName" : "Size",
            "attributeName" : "Small",
            "description" : "Compact screen size"
        }
    ],
    "categoryId" : "televisions",
    "id" : "94305057-fc1f-3f88-cb74-7056e72077ac"
}