diff --git a/ChangeLog b/ChangeLog
index 5cf4489cdb..10f711de39 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Jun 21 10:46:32 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* transcode.c (enc_arg): default interanl encoding may not be set.
+	  [ruby-core:23932]
+
 Sat Jun 20 21:11:43 2009  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* numeric.c (num_div): don't use num_floor which is actually
diff --git a/transcode.c b/transcode.c
index 7fb25601fc..ee6794d9be 100644
--- a/transcode.c
+++ b/transcode.c
@@ -2470,13 +2470,13 @@ enc_arg(volatile VALUE *arg, const char **name_p, rb_encoding **enc_p)
     int encidx;
     VALUE encval;
 
-    if ((encidx = rb_to_encoding_index(encval = *arg)) < 0) {
+    if (((encidx = rb_to_encoding_index(encval = *arg)) < 0) ||
+	!(enc = rb_enc_from_index(encidx))) {
 	enc = NULL;
 	encidx = 0;
 	n = StringValueCStr(*arg);
     }
     else {
-	enc = rb_enc_from_index(encidx);
 	n = rb_enc_name(enc);
     }