title: React-Redux: Actions must be plain objects. Use custom middleware for async actions
body: <blockquote>
    <p>Unhandled Rejection (Error): Actions must be plain objects. Use custom middleware for async actions.</p>
  </blockquote>
  
  <p>I wanted to add comments with every posts. So when fetch posts are run I want to call fetch comment API for all post.</p>
  
  <pre><code>export function bindComments(postId) {
    return API.fetchComments(postId).then(comments =&gt; {
      return {
        type: BIND_COMMENTS,
        comments,
        postId
      }
    })
  }
  </code></pre>