diff --git a/ChangeLog b/ChangeLog
index ff74219b0b..1768f6b581 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Nov 21 04:34:16 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* symbol.c (rb_str_intern): should not freeze the receiver itself
+	  unexpectedly.  [ruby-core:71611] [Bug #11721]
+
 Fri Nov 20 23:15:18 2015  Naotoshi Seo  <sonots@gmail.com>
 
 	* lib/logger.rb: expose logger mutex
diff --git a/symbol.c b/symbol.c
index 4fe3adf724..65ffab0958 100644
--- a/symbol.c
+++ b/symbol.c
@@ -674,13 +674,14 @@ rb_str_intern(VALUE str)
 #if USE_SYMBOL_GC
     enc = rb_enc_get(str);
     ascii = rb_usascii_encoding();
-    if (enc != ascii) {
-	if (sym_check_asciionly(str)) {
-	    str = rb_str_dup(str);
-	    rb_enc_associate(str, ascii);
-	    OBJ_FREEZE(str);
-	    enc = ascii;
-	}
+    if (enc != ascii && sym_check_asciionly(str)) {
+	str = rb_str_dup(str);
+	rb_enc_associate(str, ascii);
+	OBJ_FREEZE(str);
+	enc = ascii;
+    }
+    else {
+	str = rb_str_new_frozen(str);
     }
     str = rb_fstring(str);
     type = rb_str_symname_type(str, IDSET_ATTRSET_FOR_INTERN);
diff --git a/test/ruby/test_symbol.rb b/test/ruby/test_symbol.rb
index 64ddf3fcf5..a7ffe7b8b9 100644
--- a/test/ruby/test_symbol.rb
+++ b/test/ruby/test_symbol.rb
@@ -336,4 +336,12 @@ class TestSymbol < Test::Unit::TestCase
       }
     end;
   end
+
+  def test_not_freeze
+    bug11721 = '[ruby-core:71611] [Bug #11721]'
+    str = "\u{1f363}".taint
+    assert_not_predicate(str, :frozen?)
+    assert_equal str, str.to_sym.to_s
+    assert_not_predicate(str, :frozen?, bug11721)
+  end
 end