Ambiguous multiple inheritance of template classes

I've got a real situation which can be summarized in the following example:

template< typename ListenerType >
struct Notifier
{
    void add_listener( ListenerType& ){}
};

struct TimeListener{ };
struct SpaceListener{ };

struct A : public Notifier< TimeListener >
         , public Notifier< SpaceListener >
{

};

struct B : TimeListener{ };

int main()
{
    A a;
    B b;

    a.add_listener( b );    // why is ambiguous?

    return 0;
}

Why is not obvious to the compiler that B is a TimeListener, and therefore the only possible overload resolution is Notifier< TimeListener >::add_listener( TimeListener& ) ?