diff --git a/ChangeLog b/ChangeLog
index ef4812da13..71e9e54135 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Oct  2 10:44:01 2011  Kazuki Tsujimoto  <kazuki@callcc.net>
+
+	* io.c: fix documentation of ARGF.lineno=.
+
 Sat Oct  1 20:03:19 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (have_framework): try as Objective-C.
diff --git a/io.c b/io.c
index 6bade4547f..2ffe0aebf6 100644
--- a/io.c
+++ b/io.c
@@ -6767,7 +6767,7 @@ argf_initialize_copy(VALUE argf, VALUE orig)
 
 /*
  *  call-seq:
- *     ARGF.lineno = number  -> nil
+ *     ARGF.lineno = integer  -> integer
  *
  *  Sets the line number of +ARGF+ as a whole to the given +Integer+.
  *
@@ -6780,7 +6780,7 @@ argf_initialize_copy(VALUE argf, VALUE orig)
  *      ARGF.lineno      #=> 0
  *      ARGF.readline    #=> "This is line 1\n"
  *      ARGF.lineno      #=> 1
- *      ARGF.lineno = 0  #=> nil
+ *      ARGF.lineno = 0  #=> 0
  *      ARGF.lineno      #=> 0
  */
 static VALUE