diff --git a/lib/fast_gettext.rb b/lib/fast_gettext.rb
index d794ec0..c469837 100644
--- a/lib/fast_gettext.rb
+++ b/lib/fast_gettext.rb
@@ -28,19 +28,20 @@ def self.add_text_domain(name, options)
     translation_repositories[name] = TranslationRepository.build(name, options)
   end
 
+  # deprecated, just a crutch to migrate to new api
   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
+    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
diff --git a/spec/fast_gettext/vendor/string_spec.rb b/spec/fast_gettext/vendor/string_spec.rb
index 887ec3d..6cae522 100644
--- a/spec/fast_gettext/vendor/string_spec.rb
+++ b/spec/fast_gettext/vendor/string_spec.rb
@@ -56,12 +56,11 @@ def %(*args)
   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
+  ensure # cleanup
+    eval <<~CODE
+      class ::String
+        alias :% :_fast_gettext_old_format_m
+      end
+    CODE
   end
 end