问C#中的多个MongoDb过滤器

EN

        public string MultipleFilters(string collectionName, Dictionary<string, string> dictFilters)
        { 
            var filter = Builders<BsonDocument>.Filter.Eq("", "");

            foreach (KeyValuePair<string, string> entry in dictFilters)
            {
                filter = filter & Builders<BsonDocument>.Filter.Eq(entry.Key, entry.Value);
            }

            var collection = this.database.GetCollection<BsonDocument>(collectionName);           
            var document = collection.Find(filter).First();

            return document.ToJson();
        }

System.InvalidOperationException: 'Sequence contains no elements'