diff --git a/ChangeLog b/ChangeLog
index 55a31dcca1..3839b1ae4e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Oct 27 10:02:47 2008  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* io.c (read_all): the 3rd argument maybe Qnil.
+
 Mon Oct 27 01:29:55 2008  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* test/nkf/test_nkf.rb: add a test for [ruby-dev:36909].
diff --git a/io.c b/io.c
index 125180b925..ff1c74a3e3 100644
--- a/io.c
+++ b/io.c
@@ -1556,7 +1556,8 @@ read_all(rb_io_t *fptr, long siz, VALUE str)
     int cr;
 
     if (NEED_READCONV(fptr)) {
-        if (!NIL_P(str)) rb_str_set_len(str, 0);
+        if (NIL_P(str)) str = rb_str_new(NULL, 0);
+        else rb_str_set_len(str, 0);
         make_readconv(fptr);
         while (1) {
             if (fptr->cbuf_len) {