Java Generics Wildcards Question

I'm facing some problems with Generics when using Google Guava's excellent Multimap. I have a type Handler defined as such

public interface Handler<T extends Serializable> {
    void handle(T t);
} 

In another class I've defined a multimap that maps a String to a collection of Handlers.

private Multimap<String, Handler<? extends Serializable>> multimap = 
    ArrayListMultimap.create();

Now when I try to do stuff with the multimap, I'm getting compiler errors. My first attempt looked like this:

public <T extends Serializable> void doStuff1(String s, T t)  {
    Collection<Handler<T>> collection = multimap.get(s);
    for (Handler<T> handler : collection) {
        handler.handle(t);
    }
}

which resulted in the following error.

Type mismatch: cannot convert from Collection<Handler<? extends Serializable>> to Collection<Handler<T>>

Afterwards, I tried to code it like this

public void doStuff2(String s, Serializable serializable)  {
    Collection<Handler<? extends Serializable>> collection = multimap.get(s);
    for (Handler<? extends Serializable> handler : collection) {
        handler.handle(serializable); 
    }
}

which unfortunately failed as well:

The method handle(capture#1-of ? extends Serializable) in the type Handler<capture#1-of ? extends Serializable> is not applicable for the arguments (Serializable)

Any help would be greatly appreciated. Thanks.

Update:

The only way I have managed to fix this is by suppressing compiler warnings. Given the following handler:

public interface Handler<T extends Event> {
    void handle(T t);

    Class<T> getType();
}

I can write the event bus as such.