问EF代码第一次延迟加载无效？

public class Merchant : User
{
    ...

    public virtual ICollection<MerchantLocation> MerchantLocations { get; set; }
}

public class MerchantLocation : BaseEntity
{
    ...

    public int MerchantId { get; set; }
    public virtual Merchant Merchant { get; set; }       
}

public class User : BaseEntity
{
    ...
}

public class BaseEntity
{
    ...

    public int Id { get; set; }
}

public void Test_Lazy_Loading() {
    using (var context = new MyDbContext()) {
        var merchant = context.Users.OfType<Merchant>.First();
        merchant.MerchantLocations.ShouldNotBeNull(); // fails
    }
}

public void Test_Eager_Loading() {
    using (var context = new MyDbContext()) {
        var merchant = context.Users.OfType<Merchant>.Include("MerchantLocations").First();
        merchant.MerchantLocations.ShouldNotBeNull(); // passes
    }
}

Configuration.LazyLoadingEnabled = true;
Configuration.ProxyCreationEnabled = true;