title: Immutable setIn nested object returning invalid keyPath error
body: <p>I've got the following object:</p>
  
  <pre><code>const initialState = Map({
    posts: {
      most_popular: [],
      trending: [],
      latest: [],
    }
  });
  </code></pre>
  
  <p>And I'm trying to update a sub object within entries. Here is what I've tried:</p>
  
  <pre><code>intialState.setIn(["posts", "most_popular"], ["array", "of", "values"]);
  </code></pre>
  
  <p>Running this set command will return</p>
  
  <pre><code>Uncaught Error: Invalid keyPath()
  </code></pre>
  
  <p>Am I missing something? I don't need to create sub maps do I? E.g </p>
  
  <pre><code>const initialState = Map({
        posts: Map({
          most_popular: [],
  </code></pre>
  
  <p>Obviously that doesn't work (<em>I've tried</em>), but maybe there is another way of setting sub object up?</p>
  
  <p>EDIT:</p>
  
  <p>Getting the object, and updating like so works:</p>
  
  <pre><code>state.get("entries").most_popular = ["k"];
  </code></pre>
  
  <p>But this means my entries.most_popular (trending, latest) aren't immutable? Is there a way to make sub objects immutable?</p>