Java generics bug?

Let's have the following class hierarchy:

public class MyType {
}

public class MySuperclass<T extends MyType> {
    protected Map<String, String> myMap = new TreeMap<String, String>();
    protected String myMethod(String s) {
        return myMap.get(s);
    }
}

public class MySubclass extends MySuperclass {
    @Override
    protected String myMethod(String s) {
        return myMap.get(s); // <-- compilation error
    }
}

Why there is a compilation error in the overriden method of MySubclass? The error message is "Type mismatch: cannot convert from Object to String".

The interesting thing is that the compilation error dissapears if I define generics class type for MySuperclass in MySubclass definition:

public class MySubclass extends MySuperclass<MyType> {
    @Override
    protected String myMethod(String s) {
        return myMap.get(s);
    }
}

Can somebody explain this behavior? I would consider it to be a Java compiler bug.

I'm using jdk1.6.0_24.