diff --git a/ChangeLog b/ChangeLog
index 037f862aec..b1e80dab43 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Dec 25 15:42:49 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* io.c (rb_io_external_encoding): should return nil for
+	  pass-through write IO.  [ruby-dev:32740]
+
 Tue Dec 25 15:24:57 2007  Tanaka Akira  <akr@fsij.org>
 
 	* io.c (appendline): initialize rslen to 1 if rsptr is 0.
diff --git a/io.c b/io.c
index 9556b98ff6..41449beb33 100644
--- a/io.c
+++ b/io.c
@@ -5921,6 +5921,7 @@ rb_io_s_read(int argc, VALUE *argv, VALUE io)
  *     io.external_encoding   => encoding
  *
  *  Returns the Encoding object that represents the encoding of the file.
+ *  If io is write mode and no encoding is specified, returns <code>nil</code>.
  */
 
 static VALUE
@@ -5935,6 +5936,11 @@ rb_io_external_encoding(VALUE io)
     if (!fptr->enc && fptr->fd == 0) {
 	fptr->enc = rb_default_external_encoding();
     }
+    if (fptr->mode & FMODE_WRITABLE) {
+	if (fptr->enc)
+	    return rb_enc_from_encoding(fptr->enc);
+	return Qnil;
+    }
     return rb_enc_from_encoding(io_read_encoding(fptr));
 }