Qunit serializer test is returning “Cannot read property ‘push’ of null” - ember.js

Question

Ember serializer test below is failing with “Cannot read property ‘push’ of null”.
I am using Pretender mock server library. The test is failing when I'm calling a store.findRecord()
Note how there are no relationships in the assignment model/serializer, which is why it's confusing that it's throwing the following error:
Click here to see the error that's getting returned
assignment serializer:
import DS from 'ember-data';
const { JSONAPISerializer } = DS;
export default JSONAPISerializer.extend({
attrs: {
autoPassReviewerNote: 'autoPassReviewerNote',
dateOfCreation: 'date_of_creation',
displayType: 'displayType',
lastUpdate: 'last_update',
moduleItem: 'moduleItem',
submissionType: 'submissionType'
}
});
assignment model:
import DS from 'ember-data';
const {
Model,
attr
} = DS;
export default Model.extend({
title: attr('string'),
submissionType: attr('string'),
description: attr('string'),
completed: attr('boolean'),
displayType: attr('string'),
dateOfCreation: attr('string'),
lastUpdate: attr('string'),
autopass: attr('boolean'),
moduleItem: attr('object')
});
serializer test (which is failing):
import { moduleForModel, test } from 'ember-qunit';
import Pretender from 'pretender';
import Ember from 'ember';
const {
run
} = Ember;
var server;
moduleForModel('assignment', 'Unit | Serializer | assignment', {
needs: [
'serializer:assignment'
],
beforeEach: function() {
server = new Pretender(function() {
// eslint-disable-next-line ember/use-ember-get-and-set
this.get('/assignments/:id', function() {
data: {
type: 'assignment',
id: 98,
attributes: {
title: 'dfgdfg',
submissionType: 'dfgdf',
displayType: 'dfgdfg',
date_of_creation: 'sdfgsdfg',
last_update: 'fgdgd'
}
}
};
return [ 200, { 'Content-Type': 'application/json' }, JSON.stringify(response) ];
});
});
},
afterEach: function() {
server.shutdown();
}
});
test('testing assignment serializer', function(assert) {
var checkAttrSerialization = (assignment) => {
assert.equal(assignment, true);
}
let store = this.store();
run(() => {
return store.findRecord('assignment', 98).then((assignment) => checkAttrSerialization(assignment));
});
});