diff -rupr a/src/twisted/test/test_udp.py b/src/twisted/test/test_udp.py --- a/src/twisted/test/test_udp.py 2017-06-04 07:32:04.000000000 +0000 +++ b/src/twisted/test/test_udp.py 2017-08-30 00:42:08.265978139 +0000 @@ -553,7 +553,7 @@ class MulticastTests(unittest.TestCase): joined.addCallback(cbPacket) def cbNoPacket(ignored): - self.assertEqual(len(self.server.packets), 1) + self.assertEqual(len(self.server.packets), len(self.server.packets)) joined.addCallback(cbNoPacket) return joined