diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 18a44da0..9079b030 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -145,14 +145,6 @@ module ActiveModel
       adapter_class
     end
 
-    def self._root
-      @@root ||= false
-    end
-
-    def self._root=(root)
-      @@root = root
-    end
-
     def self.root_name
       name.demodulize.underscore.sub(/_serializer$/, '') if name
     end
@@ -162,7 +154,7 @@ module ActiveModel
     def initialize(object, options = {})
       @object     = object
       @options    = options
-      @root       = options[:root] || (self.class._root ? self.class.root_name : false)
+      @root       = options[:root]
       @meta       = options[:meta]
       @meta_key   = options[:meta_key]
       @scope      = options[:scope]
@@ -176,7 +168,7 @@ module ActiveModel
     end
 
     def json_key
-      if root == true || root.nil?
+      if root.nil?
         self.class.root_name
       else
         root
diff --git a/lib/active_model/serializer/adapter/flatten_json.rb b/lib/active_model/serializer/adapter/flatten_json.rb
index bbcd023a..397a53b8 100644
--- a/lib/active_model/serializer/adapter/flatten_json.rb
+++ b/lib/active_model/serializer/adapter/flatten_json.rb
@@ -8,10 +8,10 @@ module ActiveModel
           super
           @result
         end
-      end
 
-      def fragment_cache(cached_hash, non_cached_hash)
-        Json::FragmentCache.new().fragment_cache(cached_hash, non_cached_hash)
+        def root
+          false
+        end
       end
     end
   end