Index: lib/redmine/codeset_util.rb
===================================================================
--- lib/redmine/codeset_util.rb	(revision 20033)
+++ lib/redmine/codeset_util.rb	(working copy)
@@ -23,9 +23,14 @@
       end
       enc = encoding.blank? ? "UTF-8" : encoding
       if enc.casecmp("UTF-8") != 0
-        str.force_encoding(enc)
-        str = str.encode("UTF-8", :invalid => :replace,
+        begin
+          str.force_encoding(enc)
+          str = str.encode("UTF-8", :invalid => :replace,
               :undef => :replace, :replace => '?')
+        rescue Encoding::ConverterNotFoundError
+          Rails.logger.warn ">>> ConverterNotFoundError (#{enc} to UTF-8)"
+          str = replace_invalid_utf8(str)
+        end
       else
         str = replace_invalid_utf8(str)
       end
Index: test/fixtures/mail_handler/body_windows-1258.eml
===================================================================
--- test/fixtures/mail_handler/body_windows-1258.eml	(nonexistent)
+++ test/fixtures/mail_handler/body_windows-1258.eml	(working copy)
@@ -0,0 +1,14 @@
+From: John Smith <JSmith@somenet.foo>
+To: "redmine@somenet.foo" <redmine@somenet.foo>
+Subject: This is a Vietnamiese test
+Content-Type: multipart/alternative;
+  boundary="_c20d9cfa-d16a-43a3-a7e5-71da7877ab23_"
+
+--_c20d9cfa-d16a-43a3-a7e5-71da7877ab23_
+Content-Type: text/plain; charset="windows-1258"
+Content-Transfer-Encoding: quoted-printable
+
+không hợp lệ
+
+--_c20d9cfa-d16a-43a3-a7e5-71da7877ab23_--
+
Index: test/unit/lib/redmine/codeset_util_test.rb
===================================================================
--- test/unit/lib/redmine/codeset_util_test.rb	(revision 20033)
+++ test/unit/lib/redmine/codeset_util_test.rb	(working copy)
@@ -101,4 +101,12 @@
     assert_equal "UTF-8", s2.encoding.to_s
     assert_equal 'こんにち?', s2
   end
+
+  test "#to_utf8 should support not supported code page" do
+    s1 = (+'không hợp lệ').force_encoding('Windows-1258')
+    s2 = Redmine::CodesetUtil.to_utf8(s1, 'Windows-1258')
+    assert s2.valid_encoding?
+    assert_equal 'UTF-8', s2.encoding.to_s
+    assert_equal 'không hợp lệ', s2
+  end
 end
Index: test/unit/mail_handler_test.rb
===================================================================
--- test/unit/mail_handler_test.rb	(revision 20033)
+++ test/unit/mail_handler_test.rb	(working copy)
@@ -747,6 +747,15 @@
     assert_equal '고맙습니다.', issue.description
   end
 
+  def test_add_issue_with_vietnamiese_body
+    issue = submit_email(
+        'body_windows-1258.eml',
+        issue: { project: 'ecookbook' }
+    )
+    assert_kind_of Issue, issue
+    assert issue.description.include?('không hợp lệ')
+  end
+
   def test_add_issue_with_no_subject_header
     with_settings :default_language => 'en' do
       issue = submit_email(