diff --git a/ChangeLog b/ChangeLog
index 48f746b64b..a863d16c81 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Feb  2 12:41:52 2009  Shugo Maeda  <shugo@ruby-lang.org>
+
+	* complex.c (f_signbit): regard NaN as a positive value.
+	  [ruby-dev:37861].
+
 Mon Feb  2 12:39:00 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* sample/test.rb (valid_syntax?), test/ruby/test_system.rb
diff --git a/complex.c b/complex.c
index 49ce51f905..180e7d23c9 100644
--- a/complex.c
+++ b/complex.c
@@ -894,8 +894,10 @@ f_signbit(VALUE x)
 #else
       {
 	  char s[2];
+	  double f = RFLOAT_VALUE(x);
 
-	  (void)snprintf(s, sizeof s, "%.0f", RFLOAT_VALUE(x));
+	  if (isnan(f)) return Qfalse;
+	  (void)snprintf(s, sizeof s, "%.0f", f);
 	  return f_boolcast(s[0] == '-');
       }
 #endif