diff --git a/lib/bundler/rubygems_ext.rb b/lib/bundler/rubygems_ext.rb
index cb131a0185..dadac408b7 100644
--- a/lib/bundler/rubygems_ext.rb
+++ b/lib/bundler/rubygems_ext.rb
@@ -351,9 +351,8 @@ module Gem
         end
 
         def extensions_dir
-          Gem.default_ext_dir_for(base_dir) ||
-            File.join(base_dir, "extensions", ORIGINAL_LOCAL_PLATFORM,
-                      Gem.extension_api_version)
+          @extensions_dir ||=
+            Gem.default_ext_dir_for(base_dir) || File.join(base_dir, "extensions", ORIGINAL_LOCAL_PLATFORM, Gem.extension_api_version)
         end
       end
     end
diff --git a/lib/rubygems/specification.rb b/lib/rubygems/specification.rb
index 6f69ee22ce..31ef29f02f 100644
--- a/lib/rubygems/specification.rb
+++ b/lib/rubygems/specification.rb
@@ -530,13 +530,6 @@ class Gem::Specification < Gem::BasicSpecification
 
   attr_reader :required_rubygems_version
 
-  ##
-  # The version of RubyGems used to create this gem.
-  #
-  # Do not set this, it is set automatically when the gem is packaged.
-
-  attr_accessor :rubygems_version
-
   ##
   # The key used to sign this gem.  See Gem::Security for details.
 
@@ -724,6 +717,21 @@ class Gem::Specification < Gem::BasicSpecification
     @test_files = Array files
   end
 
+  ######################################################################
+  # :section: Read-only attributes
+
+  ##
+  # The version of RubyGems used to create this gem.
+
+  attr_accessor :rubygems_version
+
+  ##
+  # The path where this gem installs its extensions.
+
+  def extensions_dir
+    @extensions_dir ||= super
+  end
+
   ######################################################################
   # :section: Specification internals