diff --git a/object.c b/object.c
index 6aa9c27f9d..56afc7e99b 100644
--- a/object.c
+++ b/object.c
@@ -409,7 +409,7 @@ init_copy(VALUE dest, VALUE obj)
         break;
       case T_CLASS:
       case T_MODULE:
-        // noop: handled in class.c: rb_mod_init_copy
+        rb_mod_init_copy(dest, obj);
         break;
       case T_OBJECT:
         rb_obj_copy_ivar(dest, obj);
@@ -4571,7 +4571,6 @@ InitVM_Object(void)
     rb_define_method(rb_cModule, "<=", rb_class_inherited_p, 1);
     rb_define_method(rb_cModule, ">",  rb_mod_gt, 1);
     rb_define_method(rb_cModule, ">=", rb_mod_ge, 1);
-    rb_define_method(rb_cModule, "initialize_copy", rb_mod_init_copy, 1); /* in class.c */
     rb_define_method(rb_cModule, "to_s", rb_mod_to_s, 0);
     rb_define_alias(rb_cModule, "inspect", "to_s");
     rb_define_method(rb_cModule, "included_modules", rb_mod_included_modules, 0); /* in class.c */
diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 74541bba3f..f40817e7a1 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -259,6 +259,46 @@ class TestClass < Test::Unit::TestCase
     assert_raise(TypeError) { BasicObject.dup }
   end
 
+  def test_class_hierarchy_inside_initialize_dup_bug_21538
+    ancestors = sc_ancestors = nil
+    b = Class.new
+    b.define_singleton_method(:initialize_dup) do |x|
+      ancestors = self.ancestors
+      sc_ancestors = singleton_class.ancestors
+      super(x)
+    end
+
+    a = Class.new(b)
+
+    c = a.dup
+
+    expected_ancestors = [c, b, *Object.ancestors]
+    expected_sc_ancestors = [c.singleton_class, b.singleton_class, *Object.singleton_class.ancestors]
+    assert_equal expected_ancestors, ancestors
+    assert_equal expected_sc_ancestors, sc_ancestors
+    assert_equal expected_ancestors, c.ancestors
+    assert_equal expected_sc_ancestors, c.singleton_class.ancestors
+  end
+
+  def test_class_hierarchy_inside_initialize_clone_bug_21538
+    ancestors = sc_ancestors = nil
+    a = Class.new
+    a.define_singleton_method(:initialize_clone) do |x|
+      ancestors = self.ancestors
+      sc_ancestors = singleton_class.ancestors
+      super(x)
+    end
+
+    c = a.clone
+
+    expected_ancestors = [c,  *Object.ancestors]
+    expected_sc_ancestors = [c.singleton_class, *Object.singleton_class.ancestors]
+    assert_equal expected_ancestors, ancestors
+    assert_equal expected_sc_ancestors, sc_ancestors
+    assert_equal expected_ancestors, c.ancestors
+    assert_equal expected_sc_ancestors, c.singleton_class.ancestors
+  end
+
   def test_singleton_class
     assert_raise(TypeError) { 1.extend(Module.new) }
     assert_raise(TypeError) { 1.0.extend(Module.new) }
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 35476967d7..5805ad7d9b 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -418,9 +418,6 @@ class TestModule < Test::Unit::TestCase
     instance = klass.new
     assert_equal(:first, instance.foo)
     new_mod = Module.new { define_method(:foo) { :second } }
-    assert_raise(TypeError) do
-      mod.send(:initialize_copy, new_mod)
-    end
     4.times { GC.start }
     assert_equal(:first, instance.foo) # [BUG] unreachable
   end
@@ -435,11 +432,6 @@ class TestModule < Test::Unit::TestCase
     assert_equal([:x], m.instance_methods)
     assert_equal([:@x], m.instance_variables)
     assert_equal([:X], m.constants)
-    assert_raise(TypeError) do
-      m.module_eval do
-        initialize_copy(Module.new)
-      end
-    end
 
     m = Class.new(Module) do
       def initialize_copy(other)