Reflections.getSubTypesOf(Class) is not working in Android

I have this interface:

public interface Handler {...}

and a class that implements it:

public class DefaultHandler implements Handler {...}

Both are placed in same package: com.app.hello

In that same package, I have another class with this method (that uses Reflections library):

public int getHandlersCount() {
    Reflections reflections = new Reflections("com.app.hello");
    Set<Class<? extends EventHandler>> handlers = reflections.getSubTypesOf(EventHandler.class);
    return handlers.size();
}

However, it returns always 0. It doesn't find DefaultHandler as subtype of Handler.