Adding properties to form.values in _update function

[Schmidtchen Schleicher]

I have built a form using couchtypes and want to add the username to the document in the _update function before saving it to the database. I tried it with
editForm.values.user = req.userCtx.name;
but it returned this error:

{"error":"forbidden","reason":"user: Unexpected property - validation 2"}

Here's my code:

var templates = require('duality/templates'),
        Form = require('couchtypes/forms').Form,
        recipe = require('./types').recipe;

exports.update_recipe = function(doc,req) {
        var editForm = new Form(recipe);
        editForm.validate(req);

        if (editForm.isValid()) {
                editForm.values.user = req.userCtx.name;
                return [editForm.values, {content:"Yeah, Form valid!", title: "Validform"}];
        }
};

Is this a kanso specific error?