This is a genuine bug. For the time being you might be able to get around it by replacing the following line:

listener.AppendIncomingPacketHandler<string>("Kill all humans",
                (header, con, customObject) =>
                {
                    Console.WriteLine("\nReceived custom protobuf object from " + con);
                });

with:

listener.AppendIncomingPacketHandler<string>("Kill all humans",
                (header, con, customObject) =>
                {
                    Console.WriteLine("\nReceived custom protobuf object from " + con);
                }, options);

I’m working on a fix so that your use case works as expected. It should be released as part of 3.0.1.

Regards,
Marc