I am trying out the new Firestore by Firebase. When I run the code sample from https://firebase.google.com/docs/firestore/manage-data/add-data?authuser=0, I am getting an error.

// Add a new document with a generated id.

db.collection(cities).add({

    name: Tokyo,

    country: Japan

})

.then(function(docRef) {

    console.log(Document written with ID: , docRef.id);

})

.catch(function(error) {

    console.error(Error adding document: , error);

});

Exception caught: (FirebaseError) : Function CollectionReference.add() requires its first argument to be of type object, but it was: a custom Object object

Edit:
Sorry I didnt mention I am using GWT and JSNI, it's working fine without gwt