diff --git a/activesupport/lib/active_support/fork_tracker.rb b/activesupport/lib/active_support/fork_tracker.rb
index f9e112d234b..cb004f432e4 100644
--- a/activesupport/lib/active_support/fork_tracker.rb
+++ b/activesupport/lib/active_support/fork_tracker.rb
@@ -18,6 +18,11 @@ module ActiveSupport
       end
     end
 
+    module CoreExtPrivate
+      include CoreExt
+      private :fork
+    end
+
     @pid = Process.pid
     @callbacks = []
 
@@ -30,7 +35,7 @@ module ActiveSupport
       end
 
       def hook!
-        ::Object.prepend(CoreExt)
+        ::Object.prepend(CoreExtPrivate)
         ::Kernel.singleton_class.prepend(CoreExt)
         ::Process.singleton_class.prepend(CoreExt)
       end
diff --git a/activesupport/test/fork_tracker_test.rb b/activesupport/test/fork_tracker_test.rb
index 07fad110248..990cafba681 100644
--- a/activesupport/test/fork_tracker_test.rb
+++ b/activesupport/test/fork_tracker_test.rb
@@ -12,6 +12,7 @@ class ForkTrackerTest < ActiveSupport::TestCase
       write.write "forked"
     end
 
+    assert_not respond_to?(:fork)
     pid = fork do
       read.close
       write.close