diff --git a/ChangeLog b/ChangeLog
index 44412b1b60..c8212712d5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Dec 16 22:15:17 2008  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* lib/minitest/unit.rb (MiniTest::Assertions#assert_instance_of):
+	  should assert by instance_of?, not ===.  [ruby-dev:37458]
+
 Tue Dec 16 21:59:29 2008  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* lib/test/unit.rb (Test::Unit.setup_argv): ALT_SEPARATOR support.
diff --git a/lib/minitest/unit.rb b/lib/minitest/unit.rb
index 77176240c5..25cb65c54f 100644
--- a/lib/minitest/unit.rb
+++ b/lib/minitest/unit.rb
@@ -104,7 +104,7 @@ module MiniTest
       msg = message(msg) { "Expected #{mu_pp(obj)} to be an instance of #{cls}, not #{obj.class}" }
       flip = (Module === obj) && ! (Module === cls) # HACK for specs
       obj, cls = cls, obj if flip
-      assert cls === obj, msg
+      assert obj.instance_of?(obj), msg
     end
 
     def assert_kind_of cls, obj, msg = nil # TODO: merge with instance_of