Edited Issue: HttpError.ExceptionType is null [1248]

Having the following code:

return Request.CreateErrorResponse(
HttpStatusCode.BadRequest,
new ArgumentNullException("searchCriteria"));

I expect to be able to read ExceptionType back:

SearchCriteria searchCriteria = null;
HttpResponseMessage message = controller.Do(searchCriteria);

ObjectContent<HttpError> content = message.Content as ObjectContent<HttpError>;
HttpError error = (HttpError)content.Value;
string exceptionType = error.ExceptionType;

but the last line gives always `null`? Why is that?