diff --git a/ChangeLog b/ChangeLog
index 479fa5bd4f..82db94966c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Mar 11 16:57:00 2013  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* io.c (extract_binmode): raise error even if binmode and textmode
+	  don't conflict. [Bug #5918] [ruby-core:42199]
+
 Mon Mar 11 12:25:12 2013  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* Merge Onigmo d4bad41e16e3eccd97ccce6f1f96712e557c4518.
diff --git a/io.c b/io.c
index ed8ae96d54..3efc5349b5 100644
--- a/io.c
+++ b/io.c
@@ -5020,6 +5020,8 @@ extract_binmode(VALUE opthash, int *fmode)
 	if (!NIL_P(v)) {
 	    if (*fmode & FMODE_TEXTMODE)
 		rb_raise(rb_eArgError, "textmode specified twice");
+	    if (*fmode & FMODE_BINMODE)
+		rb_raise(rb_eArgError, "both textmode and binmode specified");
 	    if (RTEST(v))
 		*fmode |= FMODE_TEXTMODE;
 	}
@@ -5027,6 +5029,8 @@ extract_binmode(VALUE opthash, int *fmode)
 	if (!NIL_P(v)) {
 	    if (*fmode & FMODE_BINMODE)
 		rb_raise(rb_eArgError, "binmode specified twice");
+	    if (*fmode & FMODE_TEXTMODE)
+		rb_raise(rb_eArgError, "both textmode and binmode specified");
 	    if (RTEST(v))
 		*fmode |= FMODE_BINMODE;
 	}
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index becafaa201..24a7f16947 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -1406,9 +1406,12 @@ EOT
   end
 
   def test_both_textmode_binmode
-    assert_raise(ArgumentError) { open("not-exist", "r", :textmode=>true, :binmode=>true) }
-    assert_raise(ArgumentError) { open("not-exist", "rt", :binmode=>true) }
-    assert_raise(ArgumentError) { open("not-exist", "rb", :textmode=>true) }
+    bug5918 = '[ruby-core:42199]'
+    assert_raise(ArgumentError, bug5918) { open("not-exist", "r", :textmode=>true, :binmode=>true) }
+    assert_raise(ArgumentError, bug5918) { open("not-exist", "rt", :binmode=>true) }
+    assert_raise(ArgumentError, bug5918) { open("not-exist", "rt", :binmode=>false) }
+    assert_raise(ArgumentError, bug5918) { open("not-exist", "rb", :textmode=>true) }
+    assert_raise(ArgumentError, bug5918) { open("not-exist", "rb", :textmode=>false) }
   end
 
   def test_textmode_decode_universal_newline_read