Ember errors.add inside catch after save causes form button to not respond - express

Question

My server responds with a 400 status if a book title already exists. The ember app catches the error and does a errors.add with the attribute name and error detail. For some reason when a receive the 400 when I change the title the form submit button is dead. I found that when I eliminate the errors.add line the problem goes away.
Here is my action:
import Ember from 'ember';
import DS from 'ember-data';
export default Ember.Route.extend({
model() {
return this.store.createRecord('book');
},
setupController: function(controller, model) {
controller.set('book', model);
controller.set('errors', DS.Errors.create());
},
actions: {
createBook: function(book) {
var _this = this;
var errors = _this.controllerFor('books.new').get('errors');
book.save().then(function(book) {
_this.transitionTo('books.book', book);
}).catch(function(response) {
response.errors.forEach(function(error) {
var attribute = error.source.pointer.split('/')[3];
//This line causes the problem---> errors.add(attribute, error.detail);
});
});
}
}
});
This is the response:
res.status(400).send({
errors: [
{
status: "400",
source: { pointer: '/data/attributes/title' },
title: "Invalid Attribute",
detail: 'must be unique'
}
]
});