diff --git a/lib/i18n.rb b/lib/i18n.rb
index 565cb248..8c1c9de6 100644
--- a/lib/i18n.rb
+++ b/lib/i18n.rb
@@ -9,7 +9,7 @@ module I18n
   autoload :Locale,  'i18n/locale'
   autoload :Tests,   'i18n/tests'
 
-  RESERVED_KEYS = [:scope, :default, :separator, :resolve, :object, :fallback, :format, :cascade, :throw, :raise, :deep_interpolation]
+  RESERVED_KEYS = [:scope, :default, :separator, :resolve, :object, :fallback, :fallback_in_progress, :format, :cascade, :throw, :raise, :deep_interpolation]
   RESERVED_KEYS_PATTERN = /%\{(#{RESERVED_KEYS.join("|")})\}/
 
   module Base