diff --git a/ChangeLog b/ChangeLog
index 797a879c39..2e2202f6e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Dec 30 09:58:56 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* object.c (rb_class_inherited_p): search the corresponding
+	  ancestor to prepended module from prepending class itself.
+	  [ruby-core:72493] [Bug #11878]
+
 Wed Dec 30 09:20:00 2015  Yuki Kurihara  <co000ri@gmail.com>
 
 	* test/stringio/test_io.rb (test_flag): add assertion for error when
diff --git a/object.c b/object.c
index 65521e2146..53354e928b 100644
--- a/object.c
+++ b/object.c
@@ -1549,18 +1549,15 @@ rb_mod_eqq(VALUE mod, VALUE arg)
 VALUE
 rb_class_inherited_p(VALUE mod, VALUE arg)
 {
-    VALUE start = mod;
-
     if (mod == arg) return Qtrue;
     if (!CLASS_OR_MODULE_P(arg) && !RB_TYPE_P(arg, T_ICLASS)) {
 	rb_raise(rb_eTypeError, "compared with non class/module");
     }
-    arg = RCLASS_ORIGIN(arg);
-    if (class_search_ancestor(mod, arg)) {
+    if (class_search_ancestor(mod, RCLASS_ORIGIN(arg))) {
 	return Qtrue;
     }
     /* not mod < arg; check if mod > arg */
-    if (class_search_ancestor(arg, start)) {
+    if (class_search_ancestor(arg, mod)) {
 	return Qfalse;
     }
     return Qnil;
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 55678e42a8..6b7c0da4a2 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -1566,6 +1566,12 @@ class TestModule < Test::Unit::TestCase
     end
   end
 
+  def test_prepend_CMP
+    bug11878 = '[ruby-core:72493] [Bug #11878]'
+    assert_equal(-1, C1 <=> M2)
+    assert_equal(+1, M2 <=> C1, bug11878)
+  end
+
   def test_prepend_inheritance
     bug6654 = '[ruby-core:45914]'
     a = labeled_module("a")