title: SerializationException: type not included in serializable type set
body: <p>In my Google Web Toolkit project, I got the following error:</p>
  
  <p>com.google.gwt.user.client.rpc.SerializationException: Type ‘your.class.Type’ was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.</p>
  
  <p>What are the possible causes of this error?</p>