Ember cli mirage error: patch handler cannot read property update of null

I am using ember cli mirage with my amber app, i have data defined in fixtures and using the RestSerializer, i am trying to simulate updating the attributes of a record but getting an error: The patch handler for the url api/survey-groups/[id] threw an error: cannot read property update of null

mirage/config.js

 this.patch('/survey-groups/:id', function({ surveyGroups }, request) {
let id = request.params.id;
let attrs = this.normalizedRequestAttrs();

return surveyGroups.find(id).update(attrs);

});

mirage/serializers/application.js

import { RestSerializer } from 'ember-cli-mirage';
export default RestSerializer.extend({
primaryKey: 'keyId'});

app/serializers/application.js

import DS from 'ember-data';

export default DS.RESTSerializer.extend({
primaryKey: 'keyId', });

sample of fixture; mirage/fixtures/survey-groups.js

export default [
  {
    "code": "dfdj", 
    "description": "", 
    "keyId": 29116, 
  }, 
  {...... }]

I also noticed in the data returned by the server that an id attribute was added to each record, with a string value e.g. id: "1" When i attempt to find a record using this string value in place of the id, the record is returned.

What could be causing this error and behavior