Can anyone tell me why this code deadlocks?

deadlock.rb

require 'socket'
require 'thread'

module SC
  PORT = 0x55DE
  
  def self.test
    queue = Queue.new
    sender = MessageSender.new(queue)
    sender.start
    sender.join
  end
  
  class MessageSender
    def initialize(queue)
      @queue    = queue
      @shutdown = false
      @out      = UDPSocket.open
    end
    
    def start
      @thread = Thread.new do
        loop do
          envelope = @queue.pop
          begin
            # @out.send(envelope.message.to_packet, 0, envelope.destination, PORT)
          rescue => e
            # TODO: log send error
          end
        end
      end
    end
    
    def join
      @thread.join
    end
  end
end

SC.test

# deadlock 0x100151a70: sleep:-  - message_sender.rb:24
# deadlock 0x100170358: sleep:J(0x100151a70) (main) - message_sender.rb:35
# message_sender.rb:35:in `join': Thread(0x100170358): deadlock (fatal)
# 	from message_sender.rb:35:in `join'
# 	from message_sender.rb:11:in `test'
# 	from message_sender.rb:40