Mirage server GETs data but POST fails

I have the mirage models:

// mirage/models/country.js
import { Model, belongsTo, hasMany } from 'miragejs';

export default Model.extend({
    name: '',
    iso3166_1_alpha3: '',
    capitol_city: belongsTo('city', {inverse: null}),
    cities: hasMany('city', {inverse: 'country'})
});

and:

// mirage/models/city.js
import { Model, belongsTo } from 'miragejs';

export default Model.extend({
    name: '',
    country: belongsTo('country', {inverse: 'cities'})
});

and the serializer:

// mirage/serializers/application.js
import { camelize, capitalize, underscore } from '@ember/string';
import { JSONAPISerializer } from 'miragejs';

export default class ApplicationSerializer extends JSONAPISerializer
{
    alwaysIncludeLinkageData = true;

    keyForAttribute(attr) {
        return underscore(attr);
    };
    keyForRelationship(modelName) {
        return underscore(modelName);
    };
    typeKeyForModel(model) {
        return capitalize(camelize(model.modelName));
    };
};

When I run the tests:

import { module, test } from 'qunit';
import { setupTest } from 'ember-qunit';
import { setupMirage } from 'ember-cli-mirage/test-support';

module('Unit | Mirage | mirage models', function (hooks) {
  setupTest(hooks);
  setupMirage(hooks);

  test('it retrieves the country', async function (assert) {
    const server = this.server;
    let city = server.create('city', { id: '1', name: 'Paris' });

    server.create(
        'country',
        {
            id: 'FR',
            name: 'France',
            iso3166_1_alpha3: 'FRA',
            capitol_city: city
        }
    );

    let response = await fetch('/api/countries')
    assert.strictEqual(response.status, 200, "Should have created the model");
    let json = await response.json();
    assert.deepEqual(
        json,
        {
            data: [
                {
                    type: 'Country',
                    id: 'FR',
                    attributes: {
                        name: 'France',
                        iso3166_1_alpha3: 'FRA',
                    },
                    relationships: {
                        capitol_city: {data: {type: 'City', id: '1'}},
                        cities: {data: []},
                    }
                }
            ]
        }
    )
  });

  test('it creates the country', async function (assert) {
    const server = this.server;
    server.create('city', { id: '1', name: 'Paris' });

    let response = await fetch(
        '/api/countries',
        {
            method: 'POST',
            headers: {'Countent-Type': 'application/json'},
            body: JSON.stringify(
                {
                    data: {
                        id: 'FR',
                        type: 'Country',
                        attributes: {
                            iso3166_1_alpha3: 'FRA',
                            name: 'France',

                        },
                        relationships: {
                            capitol_city: { data: { type: 'City', id: '1'} },
                            cities: { data: [{ type: 'City', id: '1'}] }
                        }
                    }
                }
            )
        }
    );

    console.log((await response.json()).message);
    assert.strictEqual(response.status, 201, "Should have created the model");
  });
});

The first one passes and the second one fails with the message:

Mirage: You're passing the relationship 'capitol_city' to the 'country' model via a POST to '/api/countries', but you did not define the 'capitol_city' association on the 'country' model.

How can I get Mirage to recognise the capitol_city attribute on the model?