diff --git a/ChangeLog b/ChangeLog
index a1362a6045..9aec88d019 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Mon Oct 24 10:19:44 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* complex.c (Init_Complex): undefine Complex#clamp, which does not
+	  work like other Comparable methods, because Complex does not
+	  have <=> method.  patched by Tim Peters <zomg.tim AT gmail.com>
+	  in [ruby-core:77720].  [Bug #12866]
+
 Sun Oct 23 11:41:41 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* include/ruby/encoding.h: include "ruby/ruby.h" explicitly for
diff --git a/complex.c b/complex.c
index 7a9c56d667..ad1ea8514e 100644
--- a/complex.c
+++ b/complex.c
@@ -2234,6 +2234,7 @@ Init_Complex(void)
     rb_undef_method(rb_cComplex, ">");
     rb_undef_method(rb_cComplex, ">=");
     rb_undef_method(rb_cComplex, "between?");
+    rb_undef_method(rb_cComplex, "clamp");
     rb_undef_method(rb_cComplex, "div");
     rb_undef_method(rb_cComplex, "divmod");
     rb_undef_method(rb_cComplex, "floor");
diff --git a/test/ruby/test_complex.rb b/test/ruby/test_complex.rb
index 93d24a194a..2f21c10b3a 100644
--- a/test/ruby/test_complex.rb
+++ b/test/ruby/test_complex.rb
@@ -755,6 +755,7 @@ class Complex_Test < Test::Unit::TestCase
     assert_equal(false, c.respond_to?(:>))
     assert_equal(false, c.respond_to?(:>=))
     assert_equal(false, c.respond_to?(:between?))
+    assert_equal(false, c.respond_to?(:clamp))
     assert_equal(false, c.respond_to?(:div))
     assert_equal(false, c.respond_to?(:divmod))
     assert_equal(false, c.respond_to?(:floor))