diff --git a/ChangeLog b/ChangeLog
index 0e5a73cd91..4cec74f6c1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Nov 25 16:09:36 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (str_independent): no independent string points null_str.
+	  [ruby-core:20082]
+
 Tue Nov 25 03:37:42 2008  Hidetoshi NAGAI  <nagai@ai.kyutech.ac.jp>
 
 	* ext/tk/lib/tkextlib/blt/tabset.rb, 
diff --git a/string.c b/string.c
index 0c29fc9e6d..cdda09d275 100644
--- a/string.c
+++ b/string.c
@@ -471,6 +471,8 @@ rb_str_format_m(str, arg)
     return rb_str_format(1, &arg, str);
 }
 
+static const char null_str[] = "";
+
 static int
 str_independent(str)
     VALUE str;
@@ -481,6 +483,7 @@ str_independent(str)
     if (OBJ_FROZEN(str)) rb_error_frozen("string");
     if (!OBJ_TAINTED(str) && rb_safe_level() >= 4)
 	rb_raise(rb_eSecurityError, "Insecure: can't modify string");
+    if (RSTRING(str)->ptr == null_str) return 0;
     if (!FL_TEST(str, ELTS_SHARED)) return 1;
     return 0;
 }
@@ -539,7 +542,6 @@ rb_str_associated(str)
     return Qfalse;
 }
 
-static const char null_str[] = "";
 #define make_null_str(s) do { \
 	FL_SET(s, ELTS_SHARED); \
 	RSTRING(s)->ptr = (char *)null_str; \