Stack overflow error, why?

@ApiMethod(name = "saveProfile")
public Profile saveProfile(@Named("userId") String userId,
                               @Named("firstName") String firstName,
                               @Named("lastName") String lastName,
                               @Named("birthday") String birthday) {

        Profile profile = ofy().load().key(Key.create(Profile.class, userId)).now();
        if (profile == null) {
            profile = new Profile(userId, firstName, lastName, birthday);
        } else {
            profile.updateProfile(firstName, lastName, birthday);
        }
        ofy().save().entity(profile).now();
        return profile;
    }

@ApiMethod(name = "getSuggestedFriends")
public List<Profile> getSuggestedFriends(@Named("userId") String userId) {

    Profile profile = ofy().load().key(Key.create(Profile.class, userId)).now();
    if (profile != null) {
        /**
         * Queries for users with same last name as you (presumably family).
         */
        Query<Profile> sameLastName = ofy().load().type(Profile.class).order("birthday");
        sameLastName = sameLastName.filter("lastName =", profile.getLastName());

        /**
         * Gets the friends of your friends.
         */
        List<Profile> suggestedFriends = new ArrayList<>(0);
        for (Profile friend : profile.getFriends()) {
            for (Profile friendOfFriend : friend.getFriends()) {
                if (!profile.getFriends().contains(friendOfFriend))
                    suggestedFriends.add(friendOfFriend);
            }
        }

        /**
         * Merge the 2 lists into 1 list of suggested friends.
         */
        for (Profile suggestedFriend : sameLastName) {
            if (!profile.getFriends().contains(suggestedFriend))
                suggestedFriends.add(suggestedFriend);
        }
        return suggestedFriends;
    } else
        return null;
}