diff --git a/ChangeLog b/ChangeLog
index 51543e9a29..5873dac0d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jun 11 18:02:09 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (io_getc): should be 7bit if ascii.  fixes #4557
+
 Sat Jun 11 16:28:25 2011  Kouhei Sutou  <kou@cozmixng.org>
 
 	* lib/rexml/formatters/pretty.rb
diff --git a/io.c b/io.c
index dee458784a..065b165906 100644
--- a/io.c
+++ b/io.c
@@ -2851,7 +2851,7 @@ io_getc(rb_io_t *fptr, rb_encoding *enc)
 	}
 	else {
 	    io_shift_cbuf(fptr, MBCLEN_CHARFOUND_LEN(r), &str);
-	    cr = ENC_CODERANGE_VALID;
+	    cr = ISASCII(r) ? ENC_CODERANGE_7BIT : ENC_CODERANGE_VALID;
 	}
 	str = io_enc_str(str, fptr);
 	ENC_CODERANGE_SET(str, cr);
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 39e6dd6538..ec51956155 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -2047,5 +2047,14 @@ EOT
            r.close
          end)
   end
+
+  def test_getc_ascii_only
+    bug4557 = '[ruby-core:35630]'
+    c = with_tmpdir {
+      open("a", "wb") {|f| f.puts "a"}
+      open("a", "rt") {|f| f.getc}
+    }
+    assert(c.ascii_only?, "should be ascii_only #{bug4557}")
+  end
 end
 
diff --git a/version.h b/version.h
index 79e005eebf..ac7dc379d2 100644
--- a/version.h
+++ b/version.h
@@ -1,5 +1,5 @@
 #define RUBY_VERSION "1.9.2"
-#define RUBY_PATCHLEVEL 279
+#define RUBY_PATCHLEVEL 280
 #define RUBY_VERSION_MAJOR 1
 #define RUBY_VERSION_MINOR 9
 #define RUBY_VERSION_TEENY 1