diff --git a/ChangeLog b/ChangeLog
index 46332d3aaf..42128554c7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Jul 30 18:39:39 2009  Martin Duerst  <duerst@it.aoyama.ac.jp>
+
+	* transcode.c: added check for frozen string for encode! (see Bug #1836)
+
+	* test/ruby/test_transcode.rb: added tests for the above
+
 Thu Jul 30 16:45:39 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* insns.def (defineclass): preserve encoding of class/module
diff --git a/test/ruby/test_transcode.rb b/test/ruby/test_transcode.rb
index 41e032fdb7..cd956e134b 100644
--- a/test/ruby/test_transcode.rb
+++ b/test/ruby/test_transcode.rb
@@ -11,6 +11,8 @@ class TestTranscode < Test::Unit::TestCase
     assert_raise(Encoding::UndefinedConversionError) { "\x80".encode('utf-8','ASCII-8BIT') }
     assert_raise(Encoding::InvalidByteSequenceError) { "\x80".encode('utf-8','US-ASCII') }
     assert_raise(Encoding::UndefinedConversionError) { "\xA5".encode('utf-8','iso-8859-3') }
+    assert_raise(RuntimeError) { 'hello'.freeze.encode!('iso-8859-1') }
+    assert_raise(RuntimeError) { '\u3053\u3093\u306b\u3061\u306f'.freeze.encode!('iso-8859-1') } # こんにちは
   end
 
   def test_arguments
diff --git a/transcode.c b/transcode.c
index ee6794d9be..9e4c8392c6 100644
--- a/transcode.c
+++ b/transcode.c
@@ -2644,6 +2644,10 @@ str_encode_associate(VALUE str, int encidx)
 static VALUE
 str_encode_bang(int argc, VALUE *argv, VALUE str)
 {
+    if (OBJ_FROZEN(str)) { /* in future, may use str_frozen_check from string.c, but that's currently static */
+	rb_raise(rb_eRuntimeError, "string frozen");
+    }
+
     VALUE newstr = str;
     int encidx = str_transcode(argc, argv, &newstr);