title: Stuck in Immutable.js setIn method
body: <p>I just test the following code under the <a href="http://facebook.github.io/immutable-js/docs/#/Map" rel="nofollow">Immutable.js</a> docs console in Chrome version 50, but it emit a error message: <code>Uncaught Error: invalid keyPath(…)</code></p>
  
  <pre><code>var map1 = Immutable.Map({
      a: 1,
      b: {
          c: 1
      }
  });
  
  var map2 = map1.setIn(['b', 'c'], 2);
  </code></pre>
  
  <p>And then I changed the code to :</p>
  
  <pre><code>var map1 = Immutable.fromJS({
      a: 1,
      b: {
          c: 1
      }
  });
  
  var map2 = map1.setIn(['b', 'c'], 2);
  </code></pre>
  
  <p>It just works fine. But why? What's the difference between <code>fromJS</code> and <code>Map</code>?</p>