err: non-serializable object

I'm using Cacheable annotation and getting "err: Non-serializable object" from the underlying caching mechanism (memcache). My code was such as this:

public class MyResult implements Serializable
{
private MyEnum myEnum;
private AnotherObject anotherObj;
}

It is implementing Serializable why it would not work? That's because I forgot that every object in my class must also implement Serializable. So adding this to AnotherObject fixed my problem. But I did not implement Serializable in MyEnum, why it worked? It worked because enums in Java by default implement Serializable.