diff --git a/ChangeLog b/ChangeLog
index ba8a70fa7b..d467d57919 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Feb  3 12:04:47 2014  Kazuhiro NISHIYAMA  <zn@mbf.nifty.com>
+
+	* error.c: [DOC] Exception#cause may return nil. [ci skip]
+
 Mon Feb  3 10:49:48 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* dir.c (glob_helper): return the filename with actual cases on
diff --git a/error.c b/error.c
index 773d679e23..4c95fae484 100644
--- a/error.c
+++ b/error.c
@@ -806,7 +806,7 @@ rb_exc_set_backtrace(VALUE exc, VALUE bt)
 
 /*
  * call-seq:
- *   exception.cause   -> an_exception
+ *   exception.cause   -> an_exception or nil
  *
  * Returns the previous exception ($!) at the time this exception was raised.
  * This is useful for wrapping exceptions and retaining the original exception