diff --git a/ChangeLog b/ChangeLog
index ccd728c1db..1989eaca1e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Apr 14 15:58:11 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_string_value_cstr): make NUL terminated if it is
+	  not done.
+
 Wed Apr 14 12:56:21 2010  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* lib/net/imap (encode_utf7): encode & properly.  Thanks, Kengo
diff --git a/string.c b/string.c
index 1186b3b81b..e3d890cae3 100644
--- a/string.c
+++ b/string.c
@@ -1375,10 +1375,12 @@ rb_string_value_cstr(volatile VALUE *ptr)
 {
     VALUE str = rb_string_value(ptr);
     char *s = RSTRING_PTR(str);
+    long len = RSTRING_LEN(str);
 
-    if (!s || RSTRING_LEN(str) != (long)strlen(s)) {
+    if (!s || memchr(s, 0, len)) {
 	rb_raise(rb_eArgError, "string contains null byte");
     }
+    if (s[len]) rb_str_modify(str);
     return s;
 }