diff --git a/ChangeLog b/ChangeLog
index 93a705c628..4a7011329b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Jan 17 19:24:25 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* math.c (domain_check): check errno first.
+
 Sun Jan 17 14:24:35 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/iconv/iconv.c (rb_str_derive): use long.
diff --git a/math.c b/math.c
index 4d294bfbbb..bc6b7eb333 100644
--- a/math.c
+++ b/math.c
@@ -27,10 +27,10 @@ extern VALUE rb_to_float(VALUE val);
 static void
 domain_check(double x, double y, const char *msg)
 {
-    if (!isnan(y)) return;
-    else if (isnan(x)) return;
-    else {
-	if (!errno) {
+    if (!errno) {
+	if (!isnan(y)) return;
+	else if (isnan(x)) return;
+	else {
 #if defined(EDOM)
 	    errno = EDOM;
 #else