2 files changed, 7 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/core_ext/object/deep_dup.rb b/activesupport/lib/active_support/core_ext/object/deep_dup.rb
index 0191d2e973..ad5b2af161 100644
--- a/activesupport/lib/active_support/core_ext/object/deep_dup.rb
+++ b/activesupport/lib/active_support/core_ext/object/deep_dup.rb
@@ -40,6 +40,7 @@ class Hash
   #   dup[:a][:c]  # => "c"
   def deep_dup
     each_with_object(dup) do |(key, value), hash|
+      hash.delete(key)
       hash[key.deep_dup] = value.deep_dup
     end
   end
diff --git a/activesupport/test/core_ext/object/deep_dup_test.rb b/activesupport/test/core_ext/object/deep_dup_test.rb
index 91d558dbb5..38fbe1f836 100644
--- a/activesupport/test/core_ext/object/deep_dup_test.rb
+++ b/activesupport/test/core_ext/object/deep_dup_test.rb
@@ -50,4 +50,10 @@ class DeepDupTest < ActiveSupport::TestCase
     assert dup.instance_variable_defined?(:@a)
   end
 
+  def test_deep_dup_with_hash_class_key
+    hash = { Fixnum => 1 }
+    dup = hash.deep_dup
+    assert_equal dup.keys.length, 1
+  end
+
 end