Vuetify v-text-field + karma unit tests type error

Hello.spec.js

import Vue from 'vue';
import Vuex from 'vuex';
import Hello from '@/components/Hello';
import Vuetify from 'vuetify';

Vue.use(Vuetify);
Vue.use(Vuex);

describe('Hello.vue', () => {
  let store;
  const testPerson = {
    _id: '123456789',
    firstName: 'Martin',
    lastName: 'Smith'
  };
  const people = {
    namespaced: true,
    state: {
      all: [testPerson],
      name: 'people',
    },
    mutations: {
      refreshAll: (state, items) => {
    	state.all = items;
      }
    },
    actions: {
      fetchAll: ({ commit }) => {
        commit('refreshAll', [testPerson]);
      }
    }
  };

  before(() => {
    store = new Vuex.Store({
      modules: {
        people
      }
    });
  });

  it('should render empty text field', (done) => {
    const Component = Vue.extend({ ...Hello });
    const vm = new Component({
      store
    }).$mount();

    Vue.nextTick()
    .then(() => {
      console.log(vm.$el)
      vm.$el.querySelector('#search-field').value.should.eql('');
      done();
    })
    .catch(done);
  });

});

Hello.vue

<template>
  <v-layout>
    <v-card class="grey lighten-4">
      <v-card-text>
        <v-text-field
          id="search-field"
        ></v-text-field>
      </v-card-text>
    </v-card>
  </v-layout>
</template>

<script>
import { mapActions } from 'vuex';

export default {
  name: 'list',
  async created() {
    await this.fetchAll();
  },
  methods: {
    ...mapActions('people', [
      'fetchAll'
    ]),
  }
};
</script>

<style scoped>
</style>

karma.conf.js

// This is a karma config file. For more details see
//   http://karma-runner.github.io/0.13/config/configuration-file.html
// we are also using it with karma-webpack
//   https://github.com/webpack/karma-webpack

var webpackConfig = require('../../build/webpack.test.conf')

module.exports = function (config) {
  config.set({
    // to run in additional browsers:
    // 1. install corresponding karma launcher
    //    http://karma-runner.github.io/0.13/config/browsers.html
    // 2. add it to the `browsers` array below.
    browsers: ['PhantomJS'],
    frameworks: ['mocha', 'sinon-chai', 'phantomjs-shim'],
    reporters: ['spec', 'coverage'],
    files: [
      '../../node_modules/promise-polyfill/promise.min.js',  // >>> ONLY THIS LINE IS ADDED <<<
      './index.js'
    ],
    preprocessors: {
      './index.js': ['webpack', 'sourcemap']
    },
    webpack: webpackConfig,
    webpackMiddleware: {
      noInfo: true
    },
    coverageReporter: {
      dir: './coverage',
      reporters: [
        { type: 'lcov', subdir: '.' },
        { type: 'text-summary' }
      ]
    }
  })
}