Seems to be a bug in RavenDB

DocumentStoreTest.cs

using System.Linq;
using NUnit.Framework;
using Raven.Client.Embedded;

namespace DocumentStoreTest
{
    [TestFixture]
    public class DocumentStoreTests
    {
        //This test attempts to lookup an existing item when none exist in the
        //database. It returns null which that variable is assigned a new value
        //storing that new value, however, fails.
        [Test]
        public void DocumentStoreFailsWhenGrabbingNonExistingItemAndStoringNewOne()
        {
            using (var documentStore = new EmbeddableDocumentStore { RunInMemory = true, Identifier = "docstore1" }.Initialize())
            {

                using (var session = documentStore.OpenSession())
                {
                    TestModel testModelItem = session.Query<TestModel>().SingleOrDefault(t => t.Id == 1) ??
                                              new TestModel { Id = 1 };
                    Assert.IsNotNull(testModelItem);
                    session.Store(testModelItem);
                    session.SaveChanges();
                }

                using (var session = documentStore.OpenSession())
                {
                    var list = session.Query<TestModel>().ToList();
                    Assert.AreEqual(1, list.Count());
                }
            }
        }


        //This test adds a new item to the database and then performs a lookup on a
        //non existing item. Sets the variable to a new value and stores it.
        //works like expected
        [Test]
        public void DocumentStoreWorksWhenAddingItemAndThenGrabbingNonExistingItemAndStoringNewOne()
        {
            using (var documentStore = new EmbeddableDocumentStore { RunInMemory = true, Identifier = "docstore2" }.Initialize())
            {
                using (var session = documentStore.OpenSession())
                {
                    session.Store(new TestModel { Id = 1 });
                    session.SaveChanges();

                    TestModel testModelItem = session.Query<TestModel>().SingleOrDefault(t => t.Id == 2) ??
                                                  new TestModel { Id = 2 };
                    Assert.IsNotNull(testModelItem);
                    session.Store(testModelItem);
                    session.SaveChanges();

                    var list = session.Query<TestModel>().ToList();
                    Assert.AreEqual(2, list.Count());
                }
            }
        }

        //This test adds a new item to the database and then deletes it and then
        //performs a lookup on a non existing item. Sets the variable to a new 
        //value and stores it, however, fails.
        [Test]
        public void DocumentStoreWorksWhenAddingItemThenDeletingItAndThenGrabbingNonExistingItemAndStoringNewOne()
        {
            using (var documentStore = new EmbeddableDocumentStore { RunInMemory = true, Identifier = "docstore3" }.Initialize())
            {
                using (var session = documentStore.OpenSession())
                {
                    var deletedModel = new TestModel {Id = 1};
                    session.Store(deletedModel);
                    session.SaveChanges();

                    session.Delete(deletedModel);
                    session.SaveChanges();

                    TestModel testModelItem = session.Query<TestModel>().SingleOrDefault(t => t.Id == 2) ??
                                                  new TestModel { Id = 2 };
                    Assert.IsNotNull(testModelItem);
                    session.Store(testModelItem);
                    session.SaveChanges();

                    var list = session.Query<TestModel>().ToList();
                    Assert.AreEqual(1, list.Count());
                }
            }
        }

    }

    public class TestModel
    {
        public int Id { get; set; }
    }
}