diff --git a/lib/test/unit/parallel.rb b/lib/test/unit/parallel.rb
index 65a8264ea2..232aa44151 100644
--- a/lib/test/unit/parallel.rb
+++ b/lib/test/unit/parallel.rb
@@ -163,6 +163,7 @@ if $0 == __FILE__
   module Test
     module Unit
       class TestCase < MiniTest::Unit::TestCase
+        undef on_parallel_worker?
         def on_parallel_worker?
           true
         end