Unit Testing Z Entity Framework Plus Extensions

When using Z.EntityFramework.Plus extensions, I cannot find a way to mock/substitute what the context returns when any of the plus extensions, not just Future(), are being used. Everything I try throws a NullReferenceException within the z extensions future library.

Repository Example:

public Task<DocumentDomain> GetDocuments(int id)
{
  var futureQuery1 = _context.ReportTypes.Where(x => x.IsReadOnly).Future();
  var futureQuery2 = _context.Reports.Where(x => x.CreateDate < DateTime.Now.AddDays(-10)).Future();
  var futureQuery3 = _context.Person.DeferredFirstOrDefault(x => x.Id == id).FutureValue();

   return new DocumentDomain(futureQuery1.ToList(), futureQuery2.ToList(), futureQuery3.Value);
}

Unit Test Repository:

    [Fact]
    public async Task ShouldGetDocumentsForPerson1()
    {
       _context = Substitute.For<IMyDbContext>();
       _context.ReportTypes = new FakeDbSet();
       _context.ReportTypes.AddRange(GetFakeReportTypes());
       // ...etc..arrange for Reports and Person

       var repo = new DocumentRepository(_context);

       // CRASHES HERE ON futureQuery1 at .Future()
       var domain = await repo.GetDocuments(1); // fake id

       Assert.IsTrue(domain.Documents.Count > 0);
    }

Without using Futures it works fine. I've tried mocking out a few different ways but always get an error. Most of the time it's a "oops Z extensions encountered an error" exception.

I'm thinking it's because Z extensions is expecting a real ObjectContext but not sure how to overcome this. Has anyone experienced similar issues? Does anyone know of any solutions? Thanks.