Getting Unset document::element when retrieving element type

I am trying to use the bsoncxx library to access records from mongdb using C++. however when i try to retrieve the element type as shown in the code below i get the error shown below. Ive been going round in circles trying to figure this out, but i feel stumped.

My error msg:

   terminate called after throwing an instance of 'bsoncxx::v_noabi::exception'  
         what(): unset document::element.

My Code:

using namespace bsoncxx;

int main(int, char**) {
   //const std::uint8_t* recdata;
   //const std::uint8_t* elbytes[100000];
    mongocxx::instance inst{};
    mongocxx::client conn{mongocxx::uri{}};
    auto collection = conn["simpleemdb"]["priceCurves"];
    auto cursor = collection.find({});
    int counter =0;
    for (auto&& doc : cursor) {
        //recdata=doc.data();
        if (counter ==0){
            auto cEle= doc["curve_name"];
            type eltype= cEle.type();
        }
        counter++;
    }
}

can anyone help me understand why this is happening