diff --git a/lib/ams/inflector.rb b/lib/ams/inflector.rb
index 2c8fae73..0e91e6c1 100644
--- a/lib/ams/inflector.rb
+++ b/lib/ams/inflector.rb
@@ -35,7 +35,11 @@ module AMS
       end
 
       def pluralize(word)
-        word + "s".freeze
+        if word.end_with?("s")
+          "#{word}es"
+        else
+          "#{word}s"
+        end
       end
 
       def singularize(word)
diff --git a/test/unit/inflector_test.rb b/test/unit/inflector_test.rb
new file mode 100644
index 00000000..2d4a99d7
--- /dev/null
+++ b/test/unit/inflector_test.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+require "test_helper"
+
+module AMS
+  class Serializer
+    class InflectorTest < Test
+      def test_underscore_leaves_alone_word_without_caps_dash_nor_double_dot
+        assert_equal "active_model/errors", Inflector.underscore("active_model/errors")
+      end
+
+      def test_underscore_converts_dash_to_underscore
+        assert_equal "active_model_errors", Inflector.underscore("active_model-errors")
+      end
+
+      def test_underscore_coerces_non_string_to_string
+        assert_equal "active_model/errors", Inflector.underscore(:"active_model::errors")
+      end
+
+      def test_underscore_downcases_upcase_letters
+        assert_equal "active_model", Inflector.underscore("ACTIVE_MODEL")
+      end
+
+      def test_underscore_separates_upcase_letters_ending_in_a_number_from_following_upcase_letter_followed_by_downcase_letter_by_an_underscore
+        assert_equal "api1_api", Inflector.underscore("API1API")
+      end
+
+      def test_underscore_separates_camel_case_words_by_underscore
+        assert_equal "active_model", Inflector.underscore("ActiveModel")
+      end
+
+      def test_underscore_converts_double_dot_to_forward_slash
+        assert_equal "active_model/errors", Inflector.underscore("ActiveModel::Errors")
+      end
+
+      def test_underscore_does_not_mutate_the_original_string
+        original_string = "ActiveModel"
+        original_string_id = original_string.object_id
+        assert_equal "active_model", Inflector.underscore(original_string)
+        assert_equal "ActiveModel", original_string
+        assert_equal original_string_id, original_string.object_id
+      end
+
+      def test_pluralize_adds_s_to_word_not_ending_in_s
+        assert_equal "dogs", Inflector.pluralize("dog")
+      end
+
+      def test_pluralize_adds_es_to_word_ending_in_s
+        assert_equal "dresses", Inflector.pluralize("dress")
+      end
+
+      def test_singularize_removes_es_from_word_ending_in_es
+        assert_equal "beach", Inflector.singularize("beaches")
+      end
+
+      def test_singularize_removes_s_from_word_ending_in_s
+        assert_equal "day", Inflector.singularize("days")
+      end
+
+      def test_singularize_leaves_alone_word_not_ending_in_s
+        assert_equal "fish", Inflector.singularize("fish")
+      end
+    end
+  end
+end