LoadWith used with Nullable<int> foreign key throws exception => "Inheritance mapping is not defined for discriminator value '' "

LoadWith used with an int works fine but the usage of a Nullable<int> foreign key leads to the exception Inheritance mapping is not defined for discriminator value ‘’ in the ‘LinqToDbTest.A’ hierarchy

Exception message:
LinqToDB.Linq.LinqException was unhandled
  HResult=-2146233088
  Message=Inheritance mapping is not defined for discriminator value '' in the 'LinqToDbTest.A' hierarchy.
  Source=linq2db
  StackTrace:
       bei LinqToDB.Linq.Builder.TableBuilder.TableContext.DefaultInheritanceMappingException(Object value, Type type)
       bei lambda_method(Closure , IQueryRunner , IDataReader )
       bei LinqToDB.Linq.QueryRunner.Mapper`1.Map(IQueryRunner queryRunner, IDataReader dataReader)
       bei LinqToDB.Linq.QueryRunner.<ExecuteQuery>d__8`1.MoveNext()
       bei System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       bei System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       bei LinqToDbTest.LinqToDbIssue.LoadTest() in c:\users\documents\visual studio 2015\Projects\LinqToDbTest\LinqToDbTest\TestClass.cs:Zeile 107.
       bei LinqToDbTest.LinqToDbIssue..ctor() in c:\users\documents\visual studio 2015\Projects\LinqToDbTest\LinqToDbTest\TestClass.cs:Zeile 58.
       bei LinqToDbTest.Program.Main(String[] args) in c:\users\documents\visual studio 2015\Projects\LinqToDbTest\LinqToDbTest\Program.cs:Zeile 13.
       bei System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       bei System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       bei Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       bei System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       bei System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Steps to reproduce

    public class A
    {
        public A()
        {
            Discriminator = "A";
        }

        public int Id { get; set; }
        public string PropA { get; set; }
        public string Discriminator { get; set; }
    }

    public class B : A
    {
        public B()
        {
            Discriminator = "B";
        }

        public string PropB { get; set; }
    }

    public class C : B
    {
        public C()
        {
            Discriminator = "C";
        }

        public string PropC { get; set; }
    }

    public class Test
    {
        public int Id { get; set; }
        public int? TestAId { get; set; }
        public A TestA { get; set; }
    }

    public class LinqToDbIssue
    {
        public LinqToDbIssue()
        {
            DataConnection.DefaultSettings = new LinqToDbSettings();
            SetMappings();

            InsertData();

            // we can load all rows from A - everything is fine
            var listA = LoadA();

            // exception
            var listTest = LoadTest();
        }

        private void InsertData()
        {
            var b = new B
            {
                Id = 2,
                PropA = "Test B",
                PropB = "Test B"
            };

            var c = new C
            {
                Id = 3,
                PropA = "Test C",
                PropB = "Test C",
                PropC = "Test C"
            };

            var test = new Test
            {
                Id = 1,
                // please note the null here! It's the default I know ;-)
                TestAId = null
            };

            using (var db = new TestDataConnection())
            {
                db.GetTable<A>().Delete();
                db.GetTable<Test>().Delete();

                db.Insert(b);
                db.Insert(c);
                db.Insert(test);
            }
        }

        private List<A> LoadA()
        {
            using (var db = new TestDataConnection())
            {
                return db.GetTable<A>().ToList();
            }
        }

        private List<Test> LoadTest()
        {
            using (var db = new TestDataConnection())
            {
                return db.GetTable<Test>().LoadWith(x => x.TestA).ToList();
            }
        }

        private void SetMappings()
        {
            var mappingBuilder = MappingSchema.Default.GetFluentMappingBuilder();
            mappingBuilder.Entity<A>()
                .HasTableName("A")
                .Inheritance(x => x.Discriminator, "A", typeof(A))
                .Inheritance(x => x.Discriminator, "B", typeof(B))
                .Inheritance(x => x.Discriminator, "C", typeof(C))
                .Property(x => x.PropA).IsColumn().IsNullable().HasColumnName("PropA")
                .Property(x => x.Discriminator).IsDiscriminator().IsColumn().IsNullable(false).HasColumnName("Discriminator")
                .Property(x => x.Id).IsColumn().IsNullable(false).HasColumnName("Id");

            mappingBuilder.Entity<B>()
                .HasTableName("A")
                .Property(x => x.PropB).IsColumn().IsNullable().HasColumnName("PropB");

            mappingBuilder.Entity<C>()
                .HasTableName("A")
                .Property(x => x.PropC).IsColumn().IsNullable().HasColumnName("PropC");

            mappingBuilder.Entity<Test>()
                .HasTableName("Test")
                .Association(x => x.TestA, x => x.TestAId, x => x.Id)
                .Property(x => x.TestAId).IsColumn().IsNullable().HasColumnName("TestAId")
                .Property(x => x.TestA).IsNotColumn();
        }
    }

Environment details

linq2db version: 1.10.2

[jogibear9988]

in current veta release

[jogibear9988]

this is fixed