diff --git a/ChangeLog b/ChangeLog
index cdd3fec293..5596eed304 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Jan 30 17:01:21 2007  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* string.c (rb_str_sub_bang): calling rb_str_modify() should be just
+	  before actually modifying the string.
+	  fixed: [ruby-dev:30211] (originally reported by zunda)
+
 Tue Jan 30 13:24:06 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* numeric.c (int_pred): add Integer#pred corresponding
diff --git a/string.c b/string.c
index 7a5c7b45a3..7a27af8d77 100644
--- a/string.c
+++ b/string.c
@@ -2027,7 +2027,6 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 
     pat = get_pat(argv[0], 1);
     if (rb_reg_search(pat, str, 0, 0) >= 0) {
-	rb_str_modify(str);
 	match = rb_backref_get();
 	regs = RMATCH(match)->regs;
 
@@ -2043,6 +2042,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 	else {
 	    repl = rb_reg_regsub(repl, str, regs, pat);
 	}
+	rb_str_modify(str);
 	if (OBJ_TAINTED(repl)) tainted = 1;
 	plen = END(0) - BEG(0);
 	if (RSTRING_LEN(repl) > plen) {