Company Get (List) fails after Company Update

[AGBrown]

Environment: .NET 4.5 in VS2013
Requests captured using Fiddler
Possible (nasty?) fix: Make Company.CreatedAt a DateTime? instead of a DateTime (however see note at the end).
Issue: After updating a company using the code below, the final request to get a list of companies fails, if the updated company is in the block of companies requested

company.Tags.Add(newTag);
company.Tags.Add(oldTag);
api.CompanyRequest.Update(company);
api.CompanyRequest.Get(offset:(int?)currentBlock); // now this fails

Diagnosis:
RestSharp is actually the component throwing the exception. The company is updated with a value in the PUT of <created-at>30/05/2014 14:07:07</created-at> (for example), but is then returned in the GET xml with <created-at type="datetime" nil="true"></created-at>. The nil DateTime appears to then throw an exception within RestSharp:

Message: "String was not recognized as a valid DateTime."
Stacktrace:    at System.DateTimeParse.Parse(String s, DateTimeFormatInfo dtfi, DateTimeStyles styles)
   at System.DateTime.Parse(String s, IFormatProvider provider)
   at RestSharp.Deserializers.XmlDeserializer.Map(Object x, XElement root)
   at RestSharp.Deserializers.XmlDeserializer.CreateAndMap(Type t, XElement element)
   at RestSharp.Deserializers.XmlDeserializer.PopulateListFromElements(Type t, IEnumerable`1 elements, IList list)
   at RestSharp.Deserializers.XmlDeserializer.HandleListDerivative(Object x, XElement root, String propName, Type type)
   at RestSharp.Deserializers.XmlDeserializer.Deserialize[T](IRestResponse response)
   at RestSharp.RestClient.Deserialize[T](IRestRequest request, IRestResponse raw)

While changing Company.CreatedAt to public DateTime? CreatedAt { get; set; } allows all the companies to be returned in the block, that then creates a new issue: updating a company by api.CompanyRequest.Update(company) removes the value of CreatedAt.