supertypes returns empty list

[nalbion]

I'm able to get typeHierarchy/subtypes returning a subclass, but typeHierarchy/supertypes returns an empty list for package_a.Foo:

 package package_a;

public interface FooInterface {
    void foo(String bar);
}

public class BaseFoo implements FooInterface {
    public void foo(String bar) {
        System.out.println("BaseFoo " + bar);
    }
}

public class Foo extends BaseFoo {
    public Foo() {
        System.out.println("Foo() in package_a");   
    }

    public Foo(String bar) {
        System.out.println("Foo " + bar);
    }

    public void foo(String bar) {
        System.out.println("Foo " + bar);
    }
}

[nalbion]

typeHierarchy/subtypes is also returning empty, even though textDocument/prepareTypeHierarchy works