diff --git a/ChangeLog b/ChangeLog
index 2eb2cbecd3..cf5930520e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Oct  8 19:32:56 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_init_copy): clear PREP flag for copied IO.
+	  fixed: [ruby-dev:27371]
+
 Sat Oct  8 19:15:02 2005  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (rb_parser_malloc, rb_parser_free): manage parser stack on
diff --git a/io.c b/io.c
index 56727b790a..c7bb72cbd6 100644
--- a/io.c
+++ b/io.c
@@ -3433,7 +3433,7 @@ rb_io_init_copy(VALUE dest, VALUE io)
     rb_io_flush(io);
 
     /* copy OpenFile structure */
-    fptr->mode = orig->mode;
+    fptr->mode = orig->mode & ~FMODE_PREP;
     fptr->pid = orig->pid;
     fptr->lineno = orig->lineno;
     if (orig->path) fptr->path = strdup(orig->path);