Get offset of node within rapidjson?

I am deserializing a json string into an object using rapidjson. When I encounter an issue, not with the structure of the json, but with the content, I want to report an error stating the offset of where the problem is.

Unfortunately, unless it is a parse error, I don't see where I can get the current offset of a Value within a Document. Anyone have any ways of accomplishing this?

For example:

Document doc;
doc.Parse<0>(json.c_str());
if( doc.HasMember( "Country" ) ) {
    const Value& country_node = doc["Country"];
    if( !isValid(country_node.GetString()) )
        cout << "Invalid country specified at position " << country_node.Offset()?????
}