求助：Laravel + vue 刚搭好环境就出错。谁能帮我下？

错误：

created
[Vue warn]: Cannot find element: .a
Component mounted.
ready

view页面：

<!DOCTYPE html>
<html>
    <head>
        <script>
        window.Laravel = {!! json_encode([
            'csrfToken' => csrf_token(),
        ]) !!};
        </script>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <meta name="X-CSRF-TOKEN" content="{{csrf_token()}}">  
        <title>Laravel</title>
        <link rel="stylesheet" href="{{asset('public/css/app.css')}}">
        <script type="text/javascript" src="{{asset('public/js/app.js')}}"></script>
    </head>
    <body>
       <div class="a">
        <example></example>
       </div>
    </body>
</html>

app.js

require('./bootstrap');
import Vue from 'vue'
import Example from './components/Example.vue'
const app = new Vue({
    el: '.a',
    render: h => h(Example),
    created: function () {
        console.log('created');
    },
    mounted: function(){
        console.log('ready');
    }
});