diff --git a/ChangeLog b/ChangeLog
index 1b63e1d1d2..afd4e5d845 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,7 @@
-Thu Oct 15 14:57:58 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Thu Oct 15 15:14:15 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (io_encoding_set): get rid of parsing non-ascii string, and
+	  refine messages for invalid name encoding.
 
 	* io.c (io_reopen): unread current buffer before telling the
 	  position, for the case of reopening same file.  [ruby-dev:39479]
diff --git a/io.c b/io.c
index ccec1601dc..7ef11920ed 100644
--- a/io.c
+++ b/io.c
@@ -7500,8 +7500,7 @@ io_encoding_set(rb_io_t *fptr, VALUE v1, VALUE v2, VALUE opt)
 	enc2 = rb_to_encoding(v1);
 	tmp = rb_check_string_type(v2);
 	if (!NIL_P(tmp)) {
-	    char *p = StringValueCStr(tmp);
-	    if (*p == '-' && *(p+1) == '\0') {
+	    if (RSTRING_LEN(tmp) == 1 && RSTRING_PTR(tmp)[0] == '-') {
 		/* Special case - "-" => no transcoding */
 		enc = enc2;
 		enc2 = NULL;
@@ -7526,8 +7525,8 @@ io_encoding_set(rb_io_t *fptr, VALUE v1, VALUE v2, VALUE opt)
 	}
 	else {
 	    tmp = rb_check_string_type(v1);
-	    if (!NIL_P(tmp)) {
-                parse_mode_enc(StringValueCStr(tmp), &enc, &enc2);
+	    if (!NIL_P(tmp) && rb_enc_asciicompat(rb_enc_get(tmp))) {
+                parse_mode_enc(RSTRING_PTR(tmp), &enc, &enc2);
                 ecflags = rb_econv_prepare_opts(opt, &ecopts);
 	    }
 	    else {
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 6f5e686c42..976aae8c2e 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -220,33 +220,27 @@ EOT
   end
 
   def test_s_pipe_invalid
-    r, w = IO.pipe("utf-8", "euc-jp", :invalid=>:replace)
-    w << "\x80"
-    w.close
-    assert_equal("?", r.read)
-  ensure
-    r.close if r && !r.closed?
-    w.close if w && !w.closed?
+    with_pipe("utf-8", "euc-jp", :invalid=>:replace) {|r, w|
+      w << "\x80"
+      w.close
+      assert_equal("?", r.read)
+    }
   end
 
   def test_s_pipe_undef
-    r, w = IO.pipe("utf-8:euc-jp", :undef=>:replace)
-    w << "\ufffd"
-    w.close
-    assert_equal("?", r.read)
-  ensure
-    r.close if r && !r.closed?
-    w.close if w && !w.closed?
+    with_pipe("utf-8:euc-jp", :undef=>:replace) {|r, w|
+      w << "\ufffd"
+      w.close
+      assert_equal("?", r.read)
+    }
   end
 
   def test_s_pipe_undef_replace_string
-    r, w = IO.pipe("utf-8:euc-jp", :undef=>:replace, :replace=>"X")
-    w << "\ufffd"
-    w.close
-    assert_equal("X", r.read)
-  ensure
-    r.close if r && !r.closed?
-    w.close if w && !w.closed?
+    with_pipe("utf-8:euc-jp", :undef=>:replace, :replace=>"X") {|r, w|
+      w << "\ufffd"
+      w.close
+      assert_equal("X", r.read)
+    }
   end
 
   def test_dup
@@ -572,6 +566,11 @@ EOT
       assert_equal(eucjp, r.read)
     }
 
+    e = assert_raise(ArgumentError) {with_pipe("UTF-8", "UTF-8".encode("UTF-32BE")) {}}
+    assert_match(/invalid name encoding/, e.message)
+    e = assert_raise(ArgumentError) {with_pipe("UTF-8".encode("UTF-32BE")) {}}
+    assert_match(/invalid name encoding/, e.message)
+
     ENCS.each {|enc|
       with_pipe(enc) {|r, w|
         w << "\xc2\xa1"