diff --git a/ChangeLog b/ChangeLog
index 0c06c042b6..a3fe82cc53 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,7 +1,12 @@
+Sat Aug 13 18:35:27 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/stringio/stringio.c (strio_set_string): disallow nil.
+	  http://www.rubyist.net/~nobu/t/20050811.html#c05
+
 Sat Aug 13 08:01:59 2005  NARUSE, Yui  <naruse@ruby-lang.org>
 
-        * ext/nkf/lib/kconv.rb: Kconv.kconv is now alias of Kconv.conv
-        * ext/nkf/lib/kconv.rb: remove nkf dependend symbols fomr SYMBOL_TO_OPTION
+	* ext/nkf/lib/kconv.rb: Kconv.kconv is now alias of Kconv.conv
+	* ext/nkf/lib/kconv.rb: remove nkf dependend symbols fomr SYMBOL_TO_OPTION
 
 Fri Aug 12 17:06:53 2005  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 55678f71d8..60333a1ff9 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -96,9 +96,9 @@ get_strio(self)
 
 #define StringIO(obj) get_strio(obj)
 
-#define CLOSED(ptr) NIL_P((ptr)->string)
-#define READABLE(ptr) (!CLOSED(ptr) && ((ptr)->flags & FMODE_READABLE))
-#define WRITABLE(ptr) (!CLOSED(ptr) && ((ptr)->flags & FMODE_WRITABLE))
+#define CLOSED(ptr) (!((ptr)->flags & FMODE_READWRITE))
+#define READABLE(ptr) ((ptr)->flags & FMODE_READABLE)
+#define WRITABLE(ptr) ((ptr)->flags & FMODE_WRITABLE)
 
 static struct StringIO*
 readable(ptr)
@@ -361,10 +361,8 @@ strio_set_string(self, string)
 
     if (!OBJ_TAINTED(self)) rb_secure(4);
     ptr->flags &= ~FMODE_READWRITE;
-    if (!NIL_P(string)) {
-	StringValue(string);
-	ptr->flags = OBJ_FROZEN(string) ? FMODE_READABLE : FMODE_READWRITE;
-    }
+    StringValue(string);
+    ptr->flags = OBJ_FROZEN(string) ? FMODE_READABLE : FMODE_READWRITE;
     ptr->pos = 0;
     ptr->lineno = 0;
     return ptr->string = string;
@@ -382,7 +380,7 @@ strio_close(self)
     VALUE self;
 {
     struct StringIO *ptr = StringIO(self);
-    if (CLOSED(ptr) || !(ptr->flags & FMODE_READWRITE)) {
+    if (CLOSED(ptr)) {
 	rb_raise(rb_eIOError, "closed stream");
     }
     ptr->flags &= ~FMODE_READWRITE;