I get the following error when trying to create a hierarchy of exception classes.

Save error: NoPersonLinkException: Non-virtual and non-abstract type cannot be extended: InvalidUserUserException
 

Samples in docs don't use virtual or abstract keywords when extending exception classes.  

Please advise on syntax.

//FIRST CLASS:
public class InvalidUserException extends Exception{}

//SECOND CLASS:
public class NoPersonLinkException extends InvalidUserException{}