EF 4.1 Referential integrity error

I have the following classes:

public class Person
{
    [Key]
    public Guid Id { get; set; }

    [Required]
    public string FirstName { get; set; }

    [Required]
    public string LastName { get; set; }

    [Required]
    public string Email { get; set; }

    public virtual Survey Survey { get; set; }
}

public class Survey
{
    [Key]
    public Guid Id { get; set; }

    public bool IsFinished { get; set; }

    public virtual ICollection<UserAnswer> UserAnswers { get; set; }

    public virtual Person Person { get; set; }
}

public class UserAnswer
{
    [Key]
    public Guid Id { get; set; }

    public Guid SurveyId { get; set; }
    public virtual Survey Survey { get; set; }

    public Guid QuestionId { get; set; }
    public virtual Question Question { get; set; }

    public Guid AnswerId { get; set; }
    public virtual Answer Answer { get; set; }
 }

In my datacontext I have defined:

 modelBuilder.Entity<Survey>().HasRequired(s => s.Person).WithOptional();
 modelBuilder.Entity<Survey>().HasMany(s => s.UserAnswers).WithRequired(a => a.Survey).HasForeignKey(a => a.SurveyId).WillCascadeOnDelete(false);

Can someone tell me what I am doing wrong ?

Update:

When I execute this code:

var surveyRepository = new SurveyRepository();
foreach (var userAnswer in userAnswers)
{
    survey.UserAnswers.Add(userAnswer);
}
surveyRepository.InsertOrUpdate(survey);
surveyRepository.Save();

I get the following error:

A referential integrity constraint violation occurred: The property values that define the referential constraints are not consistent between principal and dependent objects in the relationship.