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