Deal with list of Refs in objectify

Car.java

package com.alejandrogr.gists.ofylistref;

import com.googlecode.objectify.Ref;
import com.googlecode.objectify.annotation.Entity;
import com.googlecode.objectify.annotation.Id;
import com.googlecode.objectify.annotation.Ignore;
import com.googlecode.objectify.annotation.Load;
import com.googlecode.objectify.annotation.OnLoad;
import java.util.ArrayList;
import java.util.List;
 
@Entity
class Car {
    
    @Ignore private List<Person> passengers;
    @Ignore private Person driver;
 
    @Id private Long id;
    @Load private transient Ref<Person> driverRef;    // Person is an @Entity
    @Load(Person.Passenger.class) private transient List<Ref<Person>> passengersRefs;
 
    public Car(){
        passengersRefs = new ArrayList<Ref<Person>>();
    }
    
    @OnLoad
    public void deRef() {
        if (passengersRefs != null) {
            passengers = new ArrayList<Person>();
            for (Ref<Person> passengerLoaded : passengersRefs) {
                if (passengerLoaded.isLoaded()) {
                    passengers.add(passengerLoaded.get());
                }
            }
        }
        if( driverRef != null ){ 
            driver = driverRef.get();
        }
    }
 
    public void addPassenger(Person passenger) {
        Ref<Person> ref = Ref.create(passenger.getKey());
        if (!passengersRefs.contains(ref)) {
            passengersRefs.add(ref);
        }
    }
    
    public Person getDriver() { return driverRef.get(); }
    public void setDriver(Person p_person) { driverRef = Ref.create(p_person.getKey()); }
    public List<Ref<Person>> getPassengersRefs() { return passengersRefs; }
    public void setPassengersRefs(List<Ref<Person>> p_passengersRefs) { passengersRefs = p_passengersRefs; }
    public List<Person> getPassengers() {return passengers;}
    public void setPassengers(List<Person> p_passengers) {passengers = p_passengers;}
}

OfyListRefTest.java

package com.alejandrogr.gists.ofylistref;

import junit.framework.TestCase;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;


import com.google.appengine.tools.development.testing.LocalBlobstoreServiceTestConfig;
import com.google.appengine.tools.development.testing.LocalDatastoreServiceTestConfig;
import com.google.appengine.tools.development.testing.LocalServiceTestHelper;
import com.googlecode.objectify.Key;
import com.googlecode.objectify.ObjectifyService;
import static com.googlecode.objectify.ObjectifyService.ofy;
import static junit.framework.Assert.assertEquals;
import static junit.framework.Assert.assertNotNull;


public class OfyListRefTest extends TestCase{

    private final LocalServiceTestHelper helper = new LocalServiceTestHelper(
            new LocalDatastoreServiceTestConfig().setDefaultHighRepJobPolicyUnappliedJobPercentage(1), 
            new LocalBlobstoreServiceTestConfig());

    @Override
    @Before
    public void setUp() {
        this.helper.setUp();
        
        ObjectifyService.register(Person.class);
        ObjectifyService.register(Car.class);
    }

    @Override
    @After
    public void tearDown() {
        this.helper.tearDown();
    }

    @Test
    public void testCreateCar(){

        Person driver = new Person();
        driver.setEmail("driver@email.com");
        driver.setName("Driver");
        Key<Person> driverKey = ofy().save().entity(driver).now();
        assertNotNull(driverKey);
        
        Car car = new Car();
        car.setDriver(driver);
        Key<Car> carKey = ofy().save().entity(car).now();
        
        assertNotNull(carKey);
    }
    
    @Test
    public void testCreateCarWithPassengers(){

        Person driver = new Person();
        driver.setEmail("driver@email.com");
        driver.setName("Driver");
        Key<Person> driverKey = ofy().save().entity(driver).now();
        
        Person passenger = new Person();
        passenger.setEmail("passenger@email.com");
        passenger.setName("Passenger");
        ofy().save().entity(passenger).now();
        
        Person passenger2 = new Person();
        passenger2.setEmail("passenger2@email.com");
        passenger2.setName("Passenger 2");
        ofy().save().entity(passenger2).now();
        
        Person passenger3 = new Person();
        passenger3.setEmail("passenger3@email.com");
        passenger3.setName("Passenger 3");
        ofy().save().entity(passenger3).now();
        
        Car car = new Car();
        car.setDriver(driver);
        car.addPassenger(passenger);
        car.addPassenger(passenger2);
        car.addPassenger(passenger3);
        Key<Car> carKey = ofy().save().entity(car).now();
        
        //clear ofy cache to ensure new objetc from DS
        ofy().clear();
        
        //check load without group
        Car carSaved = ofy().load().key( carKey ).now();
        
        assertNotNull(carSaved);
        assertEquals("The car dosen't have 3 passengers", 3, carSaved.getPassengersRefs().size());
        assertEquals("The passengers are loaded", 0, carSaved.getPassengers().size());
        
        //clear ofy cache to ensure new objetc from DS
        ofy().clear();
        
        //check load with group
        carSaved = ofy().load().group(Person.Passenger.class).key( carKey ).now();
        
        assertNotNull(carSaved);
        assertEquals("There are no passengers loaded", 3, carSaved.getPassengers().size());
        assertNotNull("Passenger email is empty", carSaved.getPassengers().get(1).getEmail());
    }


}

Person.java

package com.alejandrogr.gists.ofylistref;

import com.googlecode.objectify.Key;
import com.googlecode.objectify.annotation.Entity;
import com.googlecode.objectify.annotation.Id;

@Entity
public class Person {
    
    @Id private String email;
    private String name;

    
    public Key<Person> getKey() {
        return Key.create(Person.class, email);
    }
    
    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
    //empty class just for load group passenger
    public class Passenger{}
    
}

When I tried with these two methods:

public List<Ref> getPassengersRefs() { return passengersRefs; }
public void setPassengersRefs(List<Ref> p_passengersRefs) { passengersRefs = p_passengersRefs; }

I am getting this error:
Error:Execution failed for task ':backend:appengineEndpointsGetClientLibs'.
There was an error running endpoints command get-client-lib: Parameterized type com.googlecode.objectify.Ref'<'com.alejandrogr.gists.ofylistref.Person> not supported.

Any idea?