diff --git a/lib/gettext_i18n_rails/backend.rb b/lib/gettext_i18n_rails/backend.rb
index 38f8b8d..601f2ac 100644
--- a/lib/gettext_i18n_rails/backend.rb
+++ b/lib/gettext_i18n_rails/backend.rb
@@ -23,7 +23,12 @@ def translate(locale, key, options)
           interpolate(translation, options)
         else
           result = backend.translate(locale, key, options)
-          (RUBY19 and result.is_a?(String)) ? result.force_encoding("UTF-8") : result
+          if RUBY19 && result.is_a?(String)
+           result = result.dup if result.frozen?
+           result.force_encoding("UTF-8")
+          else
+            result
+          end
         end
       end
     end
diff --git a/spec/gettext_i18n_rails/backend_spec.rb b/spec/gettext_i18n_rails/backend_spec.rb
index 5cb0bd1..857e2aa 100644
--- a/spec/gettext_i18n_rails/backend_spec.rb
+++ b/spec/gettext_i18n_rails/backend_spec.rb
@@ -79,6 +79,13 @@
       subject.translate('xx', 'c', {}).should == 'd'
     end
 
+    it "passes non-gettext keys to default backend without modifying frozen translation" do
+      subject.backend.should_receive(:translate).with('xx', 'c', {}).and_return 'd'.freeze
+      # TODO track down why this is called 3 times on 1.8 (only 1 time on 1.9)
+      FastGettext.stub(:current_repository).and_return 'a'=>'b'
+      subject.translate('xx', 'c', {}).should == 'd'
+    end
+
     it 'temporarily sets the given locale' do
       FastGettext.should_receive(:set_locale).with('xx').and_return('xy')
       FastGettext.should_receive(:set_locale).with('xy').and_return('xx')