diff --git a/activesupport/lib/active_support/json/decoding.rb b/activesupport/lib/active_support/json/decoding.rb
index 5d4caa362fca621fe7fcc673757ff7d2bf6a8a6a..b4e41777244e659129ed44f67b6043078a16506f 100644
--- a/activesupport/lib/active_support/json/decoding.rb
+++ b/activesupport/lib/active_support/json/decoding.rb
@@ -9,10 +9,8 @@ class << self
       delegate :decode, :to => :backend
 
       def backend
-        @backend || begin
-          self.backend = "Yaml"
-          @backend
-        end
+        self.backend = "Yaml" unless defined?(@backend)
+        @backend
       end
 
       def backend=(name)