Java Generics: Error when using innerclass

Based on the answer: https://stackoverflow.com/a/47693998/8760211

Following Compile Error occurs, when I try to sort a list with elements of Type Student from within the class Invoker. Student is an innerclass of class University:

Type mismatch: cannot convert from ToIntFunction<University.Student> to ToIntFunction<? super T>

Code:

public class University implements Serializable {
    private List<Student> values = new ArrayList<Student>();

    public static final class Student implements Serializable {
        private int id;
        private String name;
        private String location;
        private Date date;

        Student(int id, String name, String location, Date date) {
            this.id = id;
            this.name = name;
            this.location = location;
            this.date = date;
        }

        // Problem occurs even with getters!!
}


public class Invoker implements Serializable {
    public static void main(String[] args) {
        List<University.Student> students = new ArrayList<>();

        Map<String, Integer> locationOrder = students.stream().collect(HashMap::new,
                (m, s) -> m.putIfAbsent(s.getName(), m.size()),
                (m1, m2) -> m2.keySet().forEach(l -> m1.putIfAbsent(l, m1.size())));

        students.sort(Comparator.comparingInt((University.Student s) -> locationOrder.get(s.name)).thenComparing(s -> s.id));        
    }
}

The following code snippet is marked in IDE as cause of the Compiler error: comparingInt((University.Student s) -> locationOrder.get(s.name)

If I move the innerclass into the class Invoker everything works fine!!

Any help would be much appreciated. Thanks in advance.