To reproduce:

var query = new QueryDocument("x", 1);
BsonValue bsonValue;
var ok = BsonTypeMapper.TryMapToBsonValue(query, out bsonValue);

TryMapToBsonValue knows about the BsonDocument object model, but not about the classes derived from BsonDocument (like QueryDocument).

There is a pull request from Oleg Kosmakov with a fix for this:

https://github.com/mongodb/mongo-csharp-driver/pull/133