Foreign key in Owned entity: "There is no corresponding CLR property or field"

Question

This is a tale of optional owned entities and foreign keys.

I'm working with EF 5 (code first) and I do this :

public class Parent {
   public Guid Id  { get; private set; }
   public OwnedType1? Owned1  { get; private set; }
   public OwnedType2? Owned2  { get; private set; }

   public Parent(Guid id, OwnedType1? owned1, OwnedType2? owned2) {
     Id = id; Owned1 = owned1; Owned2 = owned2;
   }
}

public class OwnedType1 {
   public Guid? OptionalExternalId  { get; private set; }

   public OwnedType1 (Guid? optionalExternalId) {
     OptionalExternalId = optionalExternalId;
   }
}

public class OwnedType2 {
   public Guid? OptionalExternalId  { get; private set; }

   public OwnedType2 (Guid? optionalExternalId) {
     OptionalExternalId = optionalExternalId;
   }
}

public class Shared {
   public Guid Id  { get; private set; }
   public Shared (Guid id) {
     Id = id;
   }
}

Now, the configuration :

//-------- for Parent ------------

       public void Configure(EntityTypeBuilder<Parent> builder) {

            builder
                .ToTable("Parents")
                .HasKey(p => p.Id);

            builder
                .OwnsOne(p => p.Owned1)
                .HasOne<Shared>()
                .WithMany()
                .HasForeignKey(x => x.OptionalExternalId);

             builder
                .OwnsOne(p => p.Owned2)
                .HasOne<Shared>()
                .WithMany()
                .HasForeignKey(x => x.OptionalExternalId);
            
       }

//-------- for OwnedType1 ------------

       // (there's no builder as they're owned and EntityTypeBuilder<Parent> is enough)

//-------- for OwnedType2 ------------

       // (there's no builder as they're owned and EntityTypeBuilder<Parent> is enough)

//-------- for Shared ---------------

       public void Configure(EntityTypeBuilder<Shared> builder) {

            builder
                .ToTable("Shareds")
                .HasKey(p => p.Id);
       }

Side note : If you're wondering why OwnedType1 and OwnedType2 don't each have a property called 'ParentId', it's because it's created implicitly by the "OwnsOne".

My problem is this :

When I create a new Migration, then OwnedType1 works like a charm, but for OwnedType2 (which is quasi-identical), I get his error :

The property 'OptionalExternalId' cannot be added to the type 'MyNameSpace.OwnedType2' because no property type was specified and there is no corresponding CLR property or field. To add a shadow state property, the property type must be specified.

I don't understand what it's complaining about. And why it's complaining only for one of them.

I know that you probably can't work it out with this simplified version of my schema, but what I'm asking is what you think it might be (follow your guts of EF guru) :

• Some missing constructor?
• Incorrect visibility on one of the fields?
• Bad navigation definition?
• A typo?
• Something tricky (like : If you're going to have TWO different entity classes having a one-to-many relation with Shared, then they can't use the same name for external key. Or I need to use a composite key. Or whatnot).