diff --git a/irctest/client_mock.py b/irctest/client_mock.py
index dd2086c..b3d5781 100644
--- a/irctest/client_mock.py
+++ b/irctest/client_mock.py
@@ -35,6 +35,8 @@ class ClientMock:
         if synchronize:
             token = "synchronize{}".format(time.monotonic())
             self.sendLine("PING {}".format(token))
+        else:
+            token = None
         got_pong = False
         data = b""
         (self.inbuffer, messages) = ([], self.inbuffer)
diff --git a/irctest/server_tests/test_lusers.py b/irctest/server_tests/test_lusers.py
index d4fbe24..b23f1da 100644
--- a/irctest/server_tests/test_lusers.py
+++ b/irctest/server_tests/test_lusers.py
@@ -132,7 +132,7 @@ class LusersUnregisteredTestCase(LusersTestCase):
 
     def _synchronize(self, client_name):
         """Synchronizes using a PING, but accept ERR_NOTREGISTERED as a response."""
-        self.sendLine(client_name, "PING")
+        self.sendLine(client_name, "PING PARAM")
         for _ in range(1000):
             msg = self.getRegistrationMessage(client_name)
             if msg.command in (ERR_NOTREGISTERED, "PONG"):