diff --git a/test/monitor/test_monitor.rb b/test/monitor/test_monitor.rb
index 8854b59ccc..9d07bf75f7 100644
--- a/test/monitor/test_monitor.rb
+++ b/test/monitor/test_monitor.rb
@@ -172,6 +172,8 @@ class TestMonitor < Test::Unit::TestCase
       assert @monitor.mon_locked?
       assert @monitor.mon_owned?
     end
+  ensure
+    th.join
   end
 
   def test_cond