diff --git a/test/generator/underscore_test.rb b/test/generator/underscore_test.rb
index f5a753ea6e..c0345772e7 100644
--- a/test/generator/underscore_test.rb
+++ b/test/generator/underscore_test.rb
@@ -19,5 +19,9 @@ def test_question_mark
         'unreadable_but_correctly_sized_inputs_return_?'
       )
     end
+
+    def test_fixnum_underscore
+      assert_equal '1_000_000', 1000000.underscore
+    end
   end
 end