Elements in iteration expect to have 'v-bind:key' directives

bug code：

<li v-for="item in items"  v-bind:class="{finished: item.isFinished}" v-on:click="toggleFinish(item)">
        {{item.label}}
      </li>

right code:

<li v-for="item in items" :key="item.label" v-bind:class="{finished: item.isFinished}" v-on:click="toggleFinish(item)">
        {{item.label}}
      </li>

增加：key=“item.label”