无法向模型对象的实例添加数据

我有两个这样的模型类

 public class PersonFullResponse
{
    public string profilePicture { get; set; }
    public string primaryMembership { get; set; }
    public List<EducationsResponse> educations { get; set; }
}
 public class EducationsResponse
{
    public string Id { get; set; }
    public string SchoolName { get; set; }
    public int StartYear { get; set; }
    public int EndYear { get; set; }
    public string Degree { get; set; }
    public object FieldOfStudy { get; set; }
}

如您所见，第一个模型也包含了第二个模型的列表。现在我要创建一个这个类的对象，并向它输入数据，像这样

   PersonFullResponse pfr = new PersonFullResponse();
                   var er = new EducationsResponse
                {
                    Degree = "",
                    SchoolName = "",
                    StartYear =11,
                    EndYear =12
                };
           pfr.educations.Add(er);

但是这会在最后一步抛出错误

    System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.

有谁能指出我做错了什么吗?

PersonFullResponse pfr = new PersonFullResponse {educations = new List<EducationsResponse>()};