diff --git a/lib/rubygems/defaults.rb b/lib/rubygems/defaults.rb
index db07681a17..90f09fc191 100644
--- a/lib/rubygems/defaults.rb
+++ b/lib/rubygems/defaults.rb
@@ -13,7 +13,7 @@ module Gem
   # An Array of the default sources that come with RubyGems
 
   def self.default_sources
-    %w[https://rubygems.org/]
+    @default_sources ||= %w[https://rubygems.org/]
   end
 
   ##
diff --git a/test/rubygems/helper.rb b/test/rubygems/helper.rb
index 51c99a1bc5..b797960ac9 100644
--- a/test/rubygems/helper.rb
+++ b/test/rubygems/helper.rb
@@ -400,8 +400,9 @@ class Gem::TestCase < Test::Unit::TestCase
     Gem::RemoteFetcher.fetcher = Gem::FakeFetcher.new
 
     @gem_repo = "http://gems.example.com/"
+    Gem.instance_variable_set :@default_sources, [@gem_repo]
+    Gem.instance_variable_set :@sources, nil
     @uri = Gem::URI.parse @gem_repo
-    Gem.sources.replace [@gem_repo]
 
     Gem.searcher = nil
     Gem::SpecFetcher.fetcher = nil
diff --git a/test/rubygems/test_gem.rb b/test/rubygems/test_gem.rb
index 49e81fcedb..74c8953904 100644
--- a/test/rubygems/test_gem.rb
+++ b/test/rubygems/test_gem.rb
@@ -586,6 +586,7 @@ class TestGem < Gem::TestCase
   end
 
   def test_self_default_sources
+    Gem.remove_instance_variable :@default_sources
     assert_equal %w[https://rubygems.org/], Gem.default_sources
   end
 
@@ -1198,6 +1199,8 @@ class TestGem < Gem::TestCase
     Gem.sources = nil
     Gem.configuration.sources = %w[http://test.example.com/]
     assert_equal %w[http://test.example.com/], Gem.sources
+  ensure
+    Gem.configuration.sources = nil
   end
 
   def test_try_activate_returns_true_for_activated_specs