AngularFire can't save object
<p>I'm trying to save an object to Firebase. I can read the object and it's properties ok but can't save it. The error callback results -> Error: Invalid record; could determine key for undefined. </p>

<pre><code>                var ref = new Firebase("https://myapp.firebaseio.com/myObjects/" + objectKey)
            var obj = $firebaseObject(ref);

            obj.$loaded().then(function () {

                console.log("Some property -&gt; " + obj.testProperty); 

                obj.testProperty= "someValue";

                obj.$save().then(function (ref) {
                    console.log("Saved !");

                }, function (error) {
                    console.log("Error:", error);
                });

            });
</code></pre>

<p>I can't see what I'm doing wrong...</p>

<p>Regards</p>