diff --git a/irb.gemspec b/irb.gemspec
index cce77e68f..af14713f6 100644
--- a/irb.gemspec
+++ b/irb.gemspec
@@ -33,7 +33,9 @@ Gem::Specification.new do |spec|
     "exe/irb",
     "irb.gemspec",
     "man/irb.1",
-  ] + Dir.chdir(File.expand_path('..', __FILE__)) { Dir.glob("lib/**/*") }
+  ] + Dir.chdir(File.expand_path('..', __FILE__)) do
+    Dir.glob("lib/**/*").map {|f| f unless File.directory?(f) }.compact
+  end
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]