diff --git a/ChangeLog b/ChangeLog
index 8c77907360..c613c36719 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Jul  3 06:19:58 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* lib/complex.rb (Numeric#arg): should return NaN for NaN.
+	  [ruby-core:24116]
+
 Wed Jul  1 15:49:13 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* eval.c (PUSH_ANCHOR, POP_ANCHOR, rb_thread_start_0): separated
diff --git a/lib/complex.rb b/lib/complex.rb
index fbf50ffece..808cecc95d 100644
--- a/lib/complex.rb
+++ b/lib/complex.rb
@@ -58,11 +58,7 @@ class Numeric
   # See Complex#arg.
   #
   def arg
-    if self >= 0
-      return 0
-    else
-      return Math::PI
-    end
+    Math.atan2!(0, self)
   end
   alias angle arg