title: Dynamodb Update Item Expression with python boto3
body: <p>I have a string field, "title". I am trying to update it with the update expression with</p>
  
  <pre><code>persontable.update_item(Key={'person_id':person_id}, UpdateExpression="SET title = UPDATED")
  </code></pre>
  
  <p>and I get</p>
  
  <pre><code>An error occurred (ValidationException) when calling the UpdateItem operation: The provided expression refers to an attribute that does not exist in the item
  </code></pre>
  
  <p>I can see the attribute "title" for that person in the AWS console. What gives?</p>