diff --git a/ChangeLog b/ChangeLog
index 55f97bbc87..e251b2d543 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jun  4 11:30:57 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* class.c (check_mix_method_i, do_mix_method_i): not mix methods
+	  renamed as nil.
+
 Sat Jun  4 04:04:41 2011  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* test/rubygems/test_gem_commands_which_command.rb:
diff --git a/class.c b/class.c
index 2a2b3ef131..e7c42daad7 100644
--- a/class.c
+++ b/class.c
@@ -745,6 +745,7 @@ check_mix_method_i(st_data_t key, st_data_t value, st_data_t arg)
     st_data_t alias;
 
     if (aliasing && st_lookup(aliasing, ID2SYM(id), &alias)) {
+	if (NIL_P(alias)) return ST_CONTINUE;
 	id = rb_to_id(alias);
     }
     if (st_lookup(argp->mtbl, id, NULL)) {
@@ -763,6 +764,7 @@ do_mix_method_i(st_data_t key, st_data_t value, st_data_t arg)
     st_data_t old, alias;
 
     if (aliasing && st_lookup(aliasing, ID2SYM(id), &alias)) {
+	if (NIL_P(alias)) return ST_CONTINUE;
 	id = rb_to_id(alias);
     }
     if (st_lookup(argp->mtbl, id, &old)) {
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index ec29530f8e..72dc34ee0c 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -1096,5 +1096,20 @@ class TestModule < Test::Unit::TestCase
         mix japanese, :address => :jp_address, :address= => :jp_address=
       }
     }
+
+    japanese_american = Class.new
+    assert_nothing_raised(ArgumentError) {
+      japanese_american.class_eval {
+        mix japanese, :address => nil, :address= => nil
+      }
+    }
+    assert_raise(NoMethodError) {
+      japanese_american.new.address
+    }
+    assert_nothing_raised(ArgumentError) {
+      japanese_american.class_eval {
+        mix american
+      }
+    }
   end
 end