Retrieve data from mongodb using C# driver

I'm using official mongodb driver for c# in my test project and i've already insert document from c# web application to mongodb. In mongo console, db.blog.find() can display entries I've inserted. but when i tried to retrieve them, .net throw a exception

"System.InvalidOperationException: ReadString can only be called when CurrentBsonType is String, not when CurrentBsonType is ObjectId."

my entity class is very simple

namespace MongoDBTest
{
    public class Blog
    {
        public String _id
        {
            get;
            set;
        }

        public String Title
        {
            get;
            set;
        }
    }
}

and this is my retrieve code

public List<Blog> List()
{
    MongoCollection collection = md.GetCollection<Blog>("blog");
    MongoCursor<Blog> cursor = collection.FindAllAs<Blog>();
    cursor.SetLimit(5);
    return cursor.ToList();
}

can anybody help me out? thanks!