DbSet Add方法不支持继承？

Question

我有以下POCO课程：

Public Class CompetitionSubscription
    Public Property ID As Long
    Public Overridable Property CompetitionID As Long
    Public Overridable Property CompetitionPlayerID As Long
    Public Property CompetitionStrength As Short?
    Public Property StrengthWithinSection As Short?
End Class

<NotMapped()>
Public Class FullCompetitionSubscription
    Inherits CompetitionSubscription

    Public Property Competition As Competition
    Public Property CompetitionPlayer As CompetitionPlayer

End Class

只有一个表格的DbContext：

Public Class CompetitionSubscriptionsDB
    Inherits DbContext

    Public Property CompetitionSubscriptions As DbSet(Of CompetitionSubscription)

End Class

我写过这样的单元测试：

<TestMethod()>
Public Sub AddSubscriptionToDatabase()

     Using db = New CompetitionSubscriptionsDB
         Dim subscription = New FullCompetitionSubscription With
                          {.CompetitionID = 1,
                           .CompetitionPlayerID = 1,
                           .CompetitionStrength = 2,
                           .StrengthWithinSection = 12}
         db.CompetitionSubscriptions.Add(DirectCast(subscription, CompetitionSubscription))
         db.SaveChanges()
     End Using

 End Sub

这失败了以下错误（翻译自荷兰语，所以可能不是确切的消息）： 未找到EntityType CompetitionSubscriptionsLib.FullCompetitionSubscription 的元数据。

由于FullCompetitionSubscription类使用<NotMapped>属性进行装饰，我可以理解错误消息，但DirectCast的要点是“卖出”FullCompetitionSubscription CompetitionSubscription的基数DbContext（如果我将subscription的类型更改为CompetitionSubscription，则测试通过。）

我哪里出错了？