Index: lib/redmine/codeset_util.rb
===================================================================
--- lib/redmine/codeset_util.rb	(revision 16111)
+++ lib/redmine/codeset_util.rb	(working copy)
@@ -6,8 +6,8 @@
       return str if str.nil?
       str.force_encoding('UTF-8')
       if ! str.valid_encoding?
-        str = str.encode("US-ASCII", :invalid => :replace,
-              :undef => :replace, :replace => '?').encode("UTF-8")
+        str = str.encode("UTF-8", :invalid => :replace,
+              :undef => :replace, :replace => '?')
       end
       str
     end
@@ -27,8 +27,8 @@
       else
         str.force_encoding("UTF-8")
         if ! str.valid_encoding?
-          str = str.encode("US-ASCII", :invalid => :replace,
-                :undef => :replace, :replace => '?').encode("UTF-8")
+          str = str.encode("UTF-8", :invalid => :replace,
+                :undef => :replace, :replace => '?')
         end
       end
       str
Index: test/unit/mail_handler_test.rb
===================================================================
--- test/unit/mail_handler_test.rb	(revision 16111)
+++ test/unit/mail_handler_test.rb	(working copy)
@@ -538,6 +538,16 @@
     assert_equal 'd8e8fca2dc0f896fd7cb4cb0031ba249', attachment.digest
   end
 
+  def test_invalid_utf8
+    issue = submit_email(
+              'invalid_utf8.eml',
+              :issue => {:project => 'ecookbook'}
+            )
+    assert_kind_of Issue, issue
+    description = "\xD0\x97\xD0\xB4\xD1\x80\xD0\xB0\xD0\xB2\xD1\x81\xD1\x82\xD0\xB2\xD1\x83\xD0\xB9\xD1\x82\xD0\xB5?".force_encoding('UTF-8')
+    assert_equal description, issue.description
+  end
+
   def test_gmail_with_attachment_ja
     issue = submit_email(
               'gmail_with_attachment_ja.eml',