Database GetDocumentExpiration returns null instead of expected date

LiteCore API Native.c4doc_getExpiration(_c4db, docId, &err2); did not return the expected long value (represent date time) instead it returns 0.

After update to LiteCore to
CE: f375a7fbbce062b7f3058a5c42de1befe8ee7dba
.Net test:

public void TestGetExpirationFromDeletedDoc()
        {
            DateTimeOffset dto3 = DateTimeOffset.UtcNow.AddSeconds(3);
            using (var doc1a = new MutableDocument("deleted_doc"))

{                 doc1a.SetInt("answer", 12);                 doc1a.SetValue("options", new[]
{ 1, 2, 3 }

);
                Db.Save(doc1a);
                Db.SetDocumentExpiration("deleted_doc", dto3).Should().Be(true);
                Db.Delete(doc1a);  
            }
            var exp = Db.GetDocumentExpiration("deleted_doc");
            exp.Should().BeSameDateAs(dto3); <-- failed exp is null value instead of expected date time value.
        }