Objectify v5: Profile is not a supported property type

I've been following Objectify's v5 tutorial and I have two classes where one is embedded in the other:

@Entity
public class User {

    @Id Long id;

    private Profile profile;

    private User(){}

    /**
     * Creates a new instance of user.
     * @param id Id of the user. If <code>null</code>, id will be automatically generated.
     * @param profile profile of the user.
     */
    public User(final Long id, @NotNull final Profile profile){

        this.id = id;
        this.profile = profile;

    }
}
public class Profile {

    private String firstName;

    private String middleName;

    private String lastName;

    private Date birthDate;

    private Profile(){}

    public Profile(@NotNull final String firstName, @NotNull final String lastName ){

        this.firstName = firstName;
        this.lastName = lastName;

    }
}

In order to test this code, I wrote the following endpoint:

@ApiMethod(name = "user.create", httpMethod = "post")
    public User createUser(@Named("id") Long id, @Named("fn") String firstName, @Named("ln") String lastName){

        Profile profile = new Profile(firstName,lastName);
        User user = new User(id,profile);

        ofy().save().entity(user).now();

        return user;
    }

but when I perform this request, I get an error:

Profile is not a supported property type.

I don't understand why I'm facing this problem because, as far as I can tell, my case is similar to the Car/Engine example given in the tutorial.

Thanks for your help.