title: Custom elements in iteration require 'v-bind:key' directives
body: <p>In my Nuxt app I have the following line that triggers the error mentioned in the title of this question:</p>
  
  <pre><code>&lt;template v-for="(project, index) in existingProjects"&gt;
      &lt;span :key="project.projectId"&gt;&lt;/span&gt;
  </code></pre>
  
  <p>I tried to have the <code>:key</code> attribute on the <code>template</code> element and I also tried to use just <code>index</code> as the key, to no avail.</p>
  
  <p>Any idea?</p>