Index: Lib/test/test_concurrent_futures.py
===================================================================
--- Lib/test/test_concurrent_futures.py	(revision 85250)
+++ Lib/test/test_concurrent_futures.py	(working copy)
@@ -73,14 +73,14 @@
     def _wait_on_event(self, handle):
         if sys.platform.startswith('win'):
             r = ctypes.windll.kernel32.WaitForSingleObject(handle, 5 * 1000)
-            assert r == 0
+            assert r == 0, 'unexpected error: %d' % ctypes.GetLastError()
         else:
             self.CALL_LOCKS[handle].wait()
 
     def _signal_event(self, handle):
         if sys.platform.startswith('win'):
             r = ctypes.windll.kernel32.SetEvent(handle)
-            assert r != 0
+            assert r != 0, 'unexpected error: %d' % ctypes.GetLastError()
         else:
             self.CALL_LOCKS[handle].set()