Почему возникает ошибка TypeError: Cannot convert a Symbol value to a string?

const app = new Vue({ 
  el: '#app', 
  methods: { 
    example(){ 
      console.log(_.cloneDeep(this)); 
    }, 
  }, 
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script> 
<script src="https://cdn.jsdelivr.net/npm/lodash@4.17.15/lodash.min.js"></script> 
<div id="app"> 
  <template> 
    <div 
      @click="example" 
    > 
      <span> 
        Test 
      </span> 
    <div> 
  </template> 
</div>