diff --git a/ChangeLog b/ChangeLog
index a2f1ebf5ee..750bd06c03 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Jul 14 17:29:20 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (rb_str_index_m): return nil if pos is out of string.
+	  [ruby-core:23660]
+
 Tue Jul 14 16:13:04 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (rb_io_initialize): check if the descriptor can be accessed
diff --git a/string.c b/string.c
index 9418bfe2ca..493ef5d87b 100644
--- a/string.c
+++ b/string.c
@@ -2569,6 +2569,8 @@ rb_str_index_m(int argc, VALUE *argv, VALUE str)
 
     switch (TYPE(sub)) {
       case T_REGEXP:
+	if (pos > str_strlen(str, STR_ENC_GET(str)))
+	    return Qnil;
 	pos = str_offset(RSTRING_PTR(str), RSTRING_END(str), pos,
 			 rb_enc_check(str, sub), single_byte_optimizable(str));