diff --git a/lib/active_model/serializer/adapter.rb b/lib/active_model/serializer/adapter.rb
index b8a8b9df..de46f12a 100644
--- a/lib/active_model/serializer/adapter.rb
+++ b/lib/active_model/serializer/adapter.rb
@@ -17,7 +17,10 @@ module ActiveModel
       end
 
       def to_json(options = {})
-        options[:fieldset] = ActiveModel::Serializer::Fieldset.new(serializer, options[:fields])
+        if fields = options.delete(:fields)
+          options[:fieldset] = ActiveModel::Serializer::Fieldset.new(fields, serializer.json_key)
+        end
+
         serializable_hash(options).to_json
       end
     end
diff --git a/lib/active_model/serializer/fieldset.rb b/lib/active_model/serializer/fieldset.rb
index e7d288f7..dc3ab857 100644
--- a/lib/active_model/serializer/fieldset.rb
+++ b/lib/active_model/serializer/fieldset.rb
@@ -2,26 +2,29 @@ module ActiveModel
   class Serializer
     class Fieldset
 
-      attr_accessor :fields, :root
+      attr_reader :fields, :root
 
-      def initialize(serializer, fields = {})
-        @root   = serializer.json_key
+      def initialize(fields, root = nil)
+        @root   = root
         @fields = parse(fields)
       end
 
       def fields_for(serializer)
         key = serializer.json_key || serializer.class.root_name
-        fields[key]
+        fields[key.to_sym]
       end
 
     private
 
       def parse(fields)
         if fields.is_a?(Hash)
-          fields.inject({}) { |h,(k,v)| h[k.to_s] = v.map(&:to_sym); h}
+          fields.inject({}) { |h,(k,v)| h[k.to_sym] = v.map(&:to_sym); h}
         elsif fields.is_a?(Array)
+          if root.nil?
+            raise ArgumentError, 'The root argument must be specified if the fileds argument is an array.'
+          end
           hash = {}
-          hash[root.to_s] = fields.map(&:to_sym)
+          hash[root.to_sym] = fields.map(&:to_sym)
           hash
         else
           {}
diff --git a/test/adapter/json_api/fieldset_test.rb b/test/adapter/json_api/fieldset_test.rb
index ded4e4a9..cc08ba11 100644
--- a/test/adapter/json_api/fieldset_test.rb
+++ b/test/adapter/json_api/fieldset_test.rb
@@ -17,7 +17,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_fields_array
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, ['title'])
+            fieldset = ActiveModel::Serializer::Fieldset.new(['title'], 'post')
 
             assert_equal(
               {:title=>"New Post", :links=>{:comments=>["1", "2"]}}, 
@@ -26,7 +26,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_hash
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:body]})
+            fieldset = ActiveModel::Serializer::Fieldset.new({post: [:body]})
 
             assert_equal(
               {:body=>"Body", :links=>{:comments=>["1", "2"]}}, 
@@ -35,7 +35,7 @@ module ActiveModel
           end
 
           def test_fieldset_with_multiple_hashes
-            fieldset = ActiveModel::Serializer::Fieldset.new(@serializer, {post: [:title], comment: [:body]})
+            fieldset = ActiveModel::Serializer::Fieldset.new({post: [:title], comment: [:body]})
 
             assert_equal(
               [{:body=>"comment one" }, {:body=>"comment two"}],
diff --git a/test/serializers/fieldset_test.rb b/test/serializers/fieldset_test.rb
new file mode 100644
index 00000000..05439177
--- /dev/null
+++ b/test/serializers/fieldset_test.rb
@@ -0,0 +1,26 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class FieldsetTest < Minitest::Test
+
+      def test_fieldset_with_hash
+        fieldset = ActiveModel::Serializer::Fieldset.new({'post' => ['id', 'title'], 'coment' => ['body']})
+
+        assert_equal(
+          {:post=>[:id, :title], :coment=>[:body]}, 
+          fieldset.fields
+        )
+      end
+
+      def test_fieldset_with_array_of_fields_and_root_name
+        fieldset = ActiveModel::Serializer::Fieldset.new(['title'], 'post')
+
+        assert_equal(
+          {:post => [:title]}, 
+          fieldset.fields
+        )
+      end
+    end
+  end
+end
\ No newline at end of file