Node unabailable

Hello.

Just I started to study couchbase with C# SDK.

My first program is as follows.


        var config = new ClientConfiguration
        {
            Servers = new List<Uri>
            {
                new Uri("http://localhost:8091/")

            }
        };

        var cluster = new Cluster(config);

        var authenticator = new PasswordAuthenticator("Administrator", "cardresearch");
        cluster.Authenticate(authenticator);
        var bucket = cluster.OpenBucket("mybucket");

        //var result = bucket.Insert("my-document-key", new { thisobject = "isempty" });
        //var document = new Document<string> { Id = "document_id", Content =  "value"  };


        //var result = bucket.Insert("my-document-key", new { thisobject = "isempty" });

        var document = new Document<dynamic> { Id = "document_id", Content = new { Some = "value" } };
        var result = bucket.Insert(document);

        Console.WriteLine(result.Status);

At the last line, result.Status shows “NodeUnavailable”

How can I fix it?

Thanks advance.