JSF 2 ManagedProperty is null

Question

I know there are loads of questions on SO regarding this exact issue, but I couldn't find a solution to my problem. I'm trying to use a @ManagedProperty in JSF 2. I used the example from this page, but mine's not working:

@ManagedProperty("#{userSession}")
private UserSession userSession;
public void setUserSession(UserSession userSession) {
    this.userSession = userSession;
}

Both the parent bean and the bean injected are session scoped. Both beans have the @ManagedBean attribute. No faces-config.xml declarations, no EJBs, no use of Spring. One thing I noticed from that example is that both the bean classes implement Serializable. Mine do not, and I'm not sure if that is making the difference.

When I use this code, I get a NullPointerException when I try to operate on userSession. However, I know a session exists, because when I use @BalusC's findBean convenience method, it works. One thing with this though, is that my code only works if I call userSession = findBean("userSession") inside the same method where I'm "doing stuff". If I initialize userSession in the bean's constructor, I get another NPE. Any ideas?