Ember: catch not working with ember-data findRecord error

I can't figure out why I can't catch this error:

Assertion Failed: You made a 'findRecord' request for a 'cart' with id '3ea1901a-56a9-11e7-a8f4-60e147bfe84c', but the adapter's response did not have any data

This is happening when trying to add an item to my cart service, but there's a problem with the API. Specifically, I am testing a security scenario where there's a token mismatch with the API. The API will send back an empty response, which it is doing, but I want ember to catch that error and trigger a addItemToNewCart function.

This is the 'add' method:

// cart methods
add(item) {

    let self = this;

    // get cart from API first.  if this fails (in catch clause) a new one should be created
    return this.get('store').findRecord('cart', get(this, 'cartObj.id')).then(response => {

        console.log("RESPONSE", response.get('cartitems'));
        // check if cart already has lineitem
        let existingLineItem = response.get('cartitems').findBy('skuid', item.skuid);
        if (existingLineItem) { // if item is already in cart, just add more
            console.log("line item in response, adding quantity");
            set(existingLineItem, 'quantity', parseInt(existingLineItem.quantity)+parseInt(item.quantity))
        } else {
            console.log("line item not in response, adding new");
            response.get('cartitems').addObject(item);
        }
        // saving persists the cart back to API
        response.save().then(newCart => {
            set(this, 'cartObj', newCart);
        });
    }).catch(e => {
        // this is not firing even though there is an error
        console.log("problem with findRecord - create a new cart and add item to it", e.message);
        self._addItemToNewCart(item);
    });

},

It appears that somehow the ember-data promise is successfully resolving, because the console.log messages I have within the then block are being printed:

I am guessing findRecord is looking locally first, finding the cart, executing the then block and an async findRecord is coming in later with an error from the API (too late for the catch block), maybe?

If that's the case, how do I say, "wait for a response from the API before doing anything and if the response is empty, call _addItemToNewCart" ?