Don't try and look for variables in non-existent values

test/unit/i18n_test.rb

@@ -30,8 +30,10 @@ class I18nTest < ActiveSupport::TestCase
           value.each do |subkey,subvalue|
 #            assert plural_keys.include?(subkey), "#{key}.#{subkey} is not a valid plural key"
 
-            subvalue.scan(/%\{(\w+)\}/) do
-              assert variables.include?($1), "#{key}.#{subkey} uses unknown interpolation variable #{$1}"
+            unless subvalue.nil?
+              subvalue.scan(/%\{(\w+)\}/) do
+                assert variables.include?($1), "#{key}.#{subkey} uses unknown interpolation variable #{$1}"
+              end
             end
           end
         else