title: AWS Amplify throwing SerializationException when trying to login
body: <p>I have a react native project which is connected to AWS mobilehub with AWS amplify. I have done all the configuration with <code>mobilehub configure</code> and pulled all the settings from aws using <code>mobilehub pull</code></p>
  
  <p>Register works perfectly fine. I am able to register user and get confirmation using </p>
  
  <pre><code>let resp = await Auth.signUp({
      username,
      password
  });
  </code></pre>
  
  <p>However for some bizarre reason, when I use <code>signIn</code> function with exactly the same parameters, I get <code>SerializationException</code></p>
  
  <pre><code>let resp = await Auth.signIn({
      username,
      password
  });
  </code></pre>
  
  <p>The error I get is as below</p>
  
  <pre><code>7:34:24 PM: Object {
  7:34:24 PM:   "code": "SerializationException",
  7:34:24 PM:   "message": "Start of structure or map found where not expected.",
  7:34:24 PM:   "name": "SerializationException",
  7:34:24 PM: }
  </code></pre>
  
  <p>How do I resolve this? There is no documentation anywhere on what this error is saying.</p>