Android : android retrofit callback generics

I have an interface like this:

@POST("/")
public void foo(Callback<MyObject> callback);

I want to be able to do something like this:

@POST("/")
public void foo(Callback<? extends MyObject> callback);

However, when I call foo(MyObjectExtended), where "MyObjectExtended" extends MyObject, I get a runtime exception, stating, that the fields of "MyObjectExtended" can't be parsed.

Is there a way to fix this?