如何在LINQ中包含父对象的字段

我正试图在LINQ查询中添加对象父级的字段，但它不起作用。以下是我的课程：

public Problem()
    {
        this.Questions = new List<Question>();
    }
    public int ProblemId { get; set; }
    public int SubTopicId { get; set; }
    public string Text { get; set; }
    public virtual SubTopic SubTopic { get; set; }
    public virtual ICollection<Question> Questions { get; set; }
}
public class Question : AuditableTable
{
    public int QuestionId { get; set; }
    public int ProblemId { get; set; }
    public virtual Problem Problem { get; set; }
    public virtual QuestionStatus QuestionStatus { get; set; }
}

 var questions = _questionsRepository
            .GetAll()
            .Include(q => q.Problem.SubTopicId)
            .Include(q => q.Answers)
            .ToList();

我得到这个错误：

System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=A specified Include path is not valid. 
  The EntityType 'Models.Contexts.Problem' does not declare a navigation 
  property with the name 'SubTopicId'.

.Include(x => x.Problem.SubTopic)