diff --git a/lib/active_model/serializer/array_serializer.rb b/lib/active_model/serializer/array_serializer.rb
index fa66e290a..a7e27a560 100644
--- a/lib/active_model/serializer/array_serializer.rb
+++ b/lib/active_model/serializer/array_serializer.rb
@@ -29,7 +29,7 @@ def json_key
         if @objects.first
           @objects.first.json_key.pluralize
         else
-          @resource.name.downcase.pluralize if @resource.try(:name)
+          @resource.name.underscore.pluralize if @resource.try(:name)
         end
       end
 
diff --git a/test/array_serializer_test.rb b/test/array_serializer_test.rb
index 259793f79..b7ee709c4 100644
--- a/test/array_serializer_test.rb
+++ b/test/array_serializer_test.rb
@@ -38,6 +38,17 @@ def test_meta_and_meta_key_attr_readers
         assert_equal @serializer.meta, "the meta"
         assert_equal @serializer.meta_key, "the meta key"
       end
+
+      def test_json_key_when_resource_is_empty
+        Array.class_eval do
+          def name
+            'PostComment'
+          end
+        end
+        @post_comments = []
+        @serializer = ArraySerializer.new(@post_comments)
+        assert_equal @serializer.json_key, "post_comments"
+      end
     end
   end
 end