Extending base class for payload class in AWS Lambda

Question

Say we have the following classes:

class ParentPayload {
 private String generic;
 public ParentPayload() {}

}

and

class ChildPayload extends Parent {
 private String childSpecific;
 public ChildPayload() {
  super();
 }

}

Then I have a lambda:

public class Lambda  implements RequestHandler<ChildPayload, Void> {
 public Void handleRequest(final ChildPayload payload) {
  //the payload does not include the generic attribute, only childSpecific
 }
}

I tried using param constructors, but Lambda only uses the empty constructor. I was expecting that Lambda would cast the request correctly to include the generic parameter as well as childSpecific