diff --git a/lib/fast_gettext.rb b/lib/fast_gettext.rb
index 86de8a89..d794ec04 100644
--- a/lib/fast_gettext.rb
+++ b/lib/fast_gettext.rb
@@ -28,6 +28,21 @@ def self.add_text_domain(name, options)
     translation_repositories[name] = TranslationRepository.build(name, options)
   end
 
+  def self.allow_invalid_keys!
+    eval(<<CODE)
+class ::String
+  alias :_fast_gettext_old_format_m :%
+  def %(*args)
+    begin
+      _fast_gettext_old_format_m(*args)
+    rescue KeyError
+      self
+    end
+  end
+end
+CODE
+  end
+
   # some repositories know where to store their locales
   def self.locale_path
     translation_repositories[text_domain].instance_variable_get(:@options)[:path]
diff --git a/lib/fast_gettext/vendor/string.rb b/lib/fast_gettext/vendor/string.rb
index fea75bc3..611388ad 100644
--- a/lib/fast_gettext/vendor/string.rb
+++ b/lib/fast_gettext/vendor/string.rb
@@ -58,19 +58,3 @@ def %(args)
     end
   end
 end
-
-# 1.9.1 if you misspell a %{key} your whole page would blow up, no thanks...
-begin
-  ("%{b}" % {:a=>'b'})
-rescue KeyError
-  class String
-    alias :_fast_gettext_old_format_m :%
-    def %(*args)
-      begin
-        _fast_gettext_old_format_m(*args)
-      rescue KeyError
-        self
-      end
-    end
-  end
-end
diff --git a/spec/fast_gettext/vendor/string_spec.rb b/spec/fast_gettext/vendor/string_spec.rb
index 6355e4d4..887ec3d5 100644
--- a/spec/fast_gettext/vendor/string_spec.rb
+++ b/spec/fast_gettext/vendor/string_spec.rb
@@ -49,7 +49,19 @@ def %(*args)
     end
   end
 
-  it "does not raise when key was not found" do
+  it "raise when key was not found" do
+    lambda { ("%{typo} xxx" % {:something=>1}) }.should raise_error(KeyError)
+  end
+
+  it "does not raise when key was not found if allow_invalid_keys! is enabled" do
+    FastGettext.allow_invalid_keys!
     ("%{typo} xxx" % {:something=>1}).should == "%{typo} xxx"
+
+    # cleanup
+    eval(<<CODE)
+class ::String
+  alias :% :_fast_gettext_old_format_m
+end
+CODE
   end
 end