diff --git a/lib/active_model_serializers/adapter/attributes.rb b/lib/active_model_serializers/adapter/attributes.rb
index af95797a..3372e2be 100644
--- a/lib/active_model_serializers/adapter/attributes.rb
+++ b/lib/active_model_serializers/adapter/attributes.rb
@@ -16,6 +16,8 @@ module ActiveModelSerializers
         self.class.transform_key_casing!(serialized_hash, instance_options)
       end
 
+      private
+
       def fields_to_fieldset(fields)
         return fields if fields.nil?
         resource_fields = []
@@ -27,7 +29,7 @@ module ActiveModelSerializers
           else fail ArgumentError, "Unknown conversion of fields to fieldset: '#{field.inspect}' in '#{fields.inspect}'"
           end
         end
-        relationship_fields.merge(serializer.json_key.to_sym => resource_fields)
+        relationship_fields.merge!(serializer.json_key.to_sym => resource_fields)
       end
     end
   end
diff --git a/test/adapter/json/fields_test.rb b/test/adapter/json/fields_test.rb
index f44a242f..04da0017 100644
--- a/test/adapter/json/fields_test.rb
+++ b/test/adapter/json/fields_test.rb
@@ -17,7 +17,7 @@ module ActiveModelSerializers
         end
 
         class PostSerializer < ActiveModel::Serializer
-          type 'posts'
+          type 'post'
           attributes :title, :body
           belongs_to :author
           has_many :comments
@@ -28,7 +28,7 @@ module ActiveModelSerializers
         end
 
         class CommentSerializer < ActiveModel::Serializer
-          type 'comments'
+          type 'comment'
           attributes :title, :body
           belongs_to :author
         end
@@ -47,7 +47,7 @@ module ActiveModelSerializers
           fields = [:title]
           hash = serializable(@post, adapter: :json, fields: fields, include: []).serializable_hash
           expected = { title: 'Title 1' }
-          assert_equal(expected, hash[:posts])
+          assert_equal(expected, hash[:post])
         end
 
         def test_fields_included
@@ -55,7 +55,7 @@ module ActiveModelSerializers
           hash = serializable(@post, adapter: :json, include: [:comments], fields: fields).serializable_hash
           expected = [{ body: @comment1.body }, { body: @comment2.body }]
 
-          assert_equal(expected, hash[:posts][:comments])
+          assert_equal(expected, hash[:post][:comments])
         end
       end
     end