Add opportunity product using REST

Hi!

I'm creating a webresource for adding a some default products to an opportunity. I've gone through the example here, but is still unsure of what properties to set on the entity to create.

    // Retrive the oppertunity, just to make sure something works....
    SDK.REST.retrieveRecord(
        opportunityId,"Opportunity",
        null, null,
        function (opportunity) {
            console.log("Retrieved the opportunity:" + opportunity.Name + ".", opportunity, opportunityId);

            // Now I want to add a new opportunityProduct with the productId given below..
            var toAdd = {};
     
            toAdd.ProductId = "e171ce8e-1ef3-e211-b252-984be17c47e4";
            toAdd.Quantity = 123;
            toAdd.ProductDescription = "Added from code - TEST123";
            toAdd.OpportunityId = opportunity.OpportunityId;

            SDK.REST.createRecord(
                toAdd,
                "OpportunityProduct",
                function (added) {
                    console.log("The OpportunityProduct was added:  YES!", added);
                },
                errorHandler
              );
        },
        errorHandler
  );

So, as you can see from my example, I have the productId of the product to add, and I have the opportunityId of the opportunity that the product should be added to. But to get the toAdd object correct..... The error I get is:

Error processing request stream. The request should be a valid top-level resource

I be really happy if someone could point me in the right direction. And maybe also give me some links to a document explaing this. (maybe I need a google crash course :-) )

Thanks for any help

Regards

Larsi