diff --git a/ChangeLog b/ChangeLog
index ecd03ab87e..57b6512b1a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Apr 30 11:48:31 2010  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
+
+	* string.c (rb_str_sub_bang): String#sub! now raises an error when
+	  called on a frozen string, even if no change is made.
+	  See [ruby-core:23657]
+
 Fri Apr 30 11:40:44 2010  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
 
 	* eval.c (make_exception, rb_obj_extend): Fix error messages in case
diff --git a/string.c b/string.c
index b4e105aa78..caca178fc4 100644
--- a/string.c
+++ b/string.c
@@ -3466,6 +3466,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
     }
 
     pat = get_pat(argv[0], 1);
+    str_modifiable(str);
     if (rb_reg_search(pat, str, 0, 0) >= 0) {
 	rb_encoding *enc;
 	int cr = ENC_CODERANGE(str);