diff --git a/ChangeLog b/ChangeLog
index 27c5c60200..1cdd7312d1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Feb 20 19:42:03 2008  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* encoding.c (rb_enc_associate_index): doesn't clear coderange
+	  when new encoding equals to old one.
+
 Wed Feb 20 19:15:38 2008  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* string.c (rb_enc_str_copy): added for wrapper for rb_enc_copy.
diff --git a/encoding.c b/encoding.c
index 15a0687281..b64688e0f5 100644
--- a/encoding.c
+++ b/encoding.c
@@ -613,6 +613,8 @@ void
 rb_enc_associate_index(VALUE obj, int idx)
 {
     enc_check_capable(obj);
+    if (rb_enc_get_index(obj) == idx)
+    	return;
     if (!ENC_CODERANGE_ASCIIONLY(obj) ||
 	!rb_enc_asciicompat(rb_enc_from_index(idx))) {
 	ENC_CODERANGE_CLEAR(obj);