While executing gem ... ["extconf.rb", ...] are not files

I'm trying to build a ruby gem around a C extension. The C extension compiles fine using the ruby extconf.rb; make; sudo make install routine, but when I try to build a gem using rake, the process terminates with the error at the bottom of this trace.

I'm using the post here, with the same directory structure, to create the gem.

What's wrong with my configuration? My gemspec and Rakefile are below the trace (the gem is called netconf).

// Trace

** Execute copy:netconf:i686-linux:1.9.2
install -c tmp/i686-linux/netconf/1.9.2/netconf.so lib/netconf/netconf.so
** Execute compile:netconf:i686-linux
** Execute compile:i686-linux
** Execute compile
** Invoke chmod (first_time)
** Execute chmod
** Execute build
rake aborted!
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    ["extconf.rb", "netconf.o", "netconf.so"] are not files

// netconf.gemspec

# -*- encoding: utf-8 -*-
$:.push File.expand_path("../lib", __FILE__)
require "netconf/version"

Gem::Specification.new do |s|
  s.name        = "netconf"
  s.version     = Netconf::VERSION
  s.authors     = ["..."]
  s.email       = ["..."]
  s.homepage    = "..."
  s.summary     = %q{A tool to access and write Ubuntu network configuration}
  s.description = %q{Uses ifconfig and other C system calls to access network configurations on a Ubuntu install.}

  s.rubyforge_project = "netconf"

  s.files         = `git ls-files`.split("\n")
  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
  s.require_paths = ["lib"]
  s.extensions    = ["ext/netconf/extconf.rb"]
end

// Rakefile

require 'rake'
require 'rake/extensiontask'
require 'bundler'

Rake::ExtensionTask.new("netconf") do |extension|
  extension.lib_dir = "lib/netconf"
end

task :chmod do