Google Collections: cant sort null with comparator

I keep getting a nullpointerexception when I try to sort users where a certain user has a null value in his profile. I was under the impression Google Collection would handle these null values, but it doesnt seem to work.

This is the code I use:

Comparator<UserModel> firstName_comparator = new Comparator<UserModel>() {
    @Override
    public int compare(UserModel c1, UserModel c2) {
        retu c1.getProfile().getFirstName().toLowerCase()
                .compareTo(c2.getProfile().getFirstName().toLowerCase());
        }
 };
 Collections.sort(users, Ordering.from(firstName_comparator).nullsLast());

This specific line throws the nullpointerexception:

.compareTo(c2.getProfile().getFirstName().toLowerCase());

Its because getProfile() is null.
How can I fix this? I want to be able to sort my users with null values.