NullReferenceException when including multiple levels of classes

I have the following classes:

public class A {
     public List<B> bList {get;set;}
     public List<X> xList {get;set;}
}

public class X {
     // data
}

public class B {
    public List<C> cList {get;set;}
    public List<D> dList {get;set;}
}

public class C {
    // data
}

// New added class D
public class D {
    // data
}

then I have a DatabaseContext with

public class DatabaseContext : DbContext {
    public DbSet<A> aList {get;set;}
}

In the past, when there was no class D, I loaded my database like this:

DatabaseContext _databaseContext = new DatabaseContext();    
List<A> localA = _databaseContext.aList.Include("xList")
                                       .Include("bList")
                                       .Include("bList.cList")
                                       .ToList();

and it worked perfectly.

Now with the added class D I tried the following:

List<A> localA = _databaseContext.aList.Include("xList")
                                       .Include("bList")
                                       .Include("bList.cList")
                                       .Include("bList.dList")
                                       .ToList();

But it throws a System.NullReferenceException: 'Object reference not set to an instance of an object.'.

If I remove either Include("bList.cList") or Include("bList.dList"), it works again, but only loads class C or class D, respectively.

As soon as I add both lines, I get an Exception.

How can I include both classes from the database?

I can rule out typos or access issues to the database, because if I only use one of them, it works.

I also tried

List<A> localA = _databaseContext.aList.Include("xList")
                                       .Include("bList.cList")
                                       .Include("bList.dList")
                                       .ToList();

and

List<A> localA = _databaseContext.aList.Include("xList")
                                       .Include(a => a.bList.Select(b => b.cList))
                                       .Include(a => a.bList.Select(b => b.dList))
                                       .ToList();

but they also throw a NullReferenceException.

StackTrace from the Exception:

   at System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.GetOtherEndOfRelationship(IEntityWrapper wrappedEntity)
   at System.Data.Entity.Core.Objects.ObjectStateManager.AddEntityToCollectionOrReference(MergeOption mergeOption, IEntityWrapper wrappedSource, AssociationEndMember sourceMember, IEntityWrapper wrappedTarget, AssociationEndMember targetMember, Boolean setIsLoaded, Boolean relationshipAlreadyExists, Boolean inKeyEntryPromotion)
   at System.Data.Entity.Core.Objects.ObjectStateManager.UpdateRelationships(ObjectContext context, MergeOption mergeOption, AssociationSet associationSet, AssociationEndMember sourceMember, IEntityWrapper wrappedSource, AssociationEndMember targetMember, IList targets, Boolean setIsLoaded)
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper.FullSpanAction[TTargetEntity](IEntityWrapper wrappedSource, IList`1 spannedEntities, AssociationEndMember targetMember)
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper.<>c__DisplayClass12_0`1.<HandleFullSpanCollection>b__0(Shaper state, List`1 spannedEntities)
   at System.Data.Entity.Core.Common.Internal.Materialization.Coordinator`1.ResetCollection(Shaper shaper)
   at System.Data.Entity.Core.Common.Internal.Materialization.Coordinator`1.ResetCollection(Shaper shaper)
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.RowNestedResultEnumerator.MaterializeRow()
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.RowNestedResultEnumerator.MoveNext()
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.ObjectQueryNestedEnumerator.TryReadToNextElement()
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.ObjectQueryNestedEnumerator.ReadElement()
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.ObjectQueryNestedEnumerator.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at DatabaseConnector.DatabaseController.Load(String connectionName)

I found a working solution, but I have no idea why it works.

If I switch positions from

List<A> localA = _databaseContext.aList.Include("xList")
                                       .Include("bList.cList")
                                       .Include("bList.dList")
                                       .ToList();

to

List<A> localA = _databaseContext.aList.Include("bList.cList")
                                       .Include("bList.dList")
                                       .Include("xList")
                                       .ToList();

then I get no Exception.

The class X has no links to other classes and simply holds two integers.