Map deserialization doesn't use Adapter and generates runtime exception on access

Given a simple POJO "Person" with a String attribute "name" and a test like

` public static void main(String[] args) {
String json = "{\"foo\" : \"bar\"}";
Map map = JsonbBuilder.create().fromJson(json, new HashMap() {}.getClass().getGenericSuperclass());
map.keySet().forEach(Person::getName);
}
`

I end up with
`Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to test.Person
at java.util.HashMap$KeySet.forEach(HashMap.java:933)
at test.Test.main(Test.java:14)`

If I register an adapter for Person, it is never called. How can a String even fit into the Person map key and only be detected at runtime?