diff --git a/irctest/server_tests/whois.py b/irctest/server_tests/whois.py
index b9c1b3c..760573c 100644
--- a/irctest/server_tests/whois.py
+++ b/irctest/server_tests/whois.py
@@ -96,7 +96,9 @@ class _WhoisTestMixin(cases.BaseServerTestCase):
                     params=[
                         "nick1",
                         "nick2",
-                        StrRe("(@#chan1 @#chan2|@#chan2 @#chan1)"),
+                        # trailing space was required by the RFCs, and Modern explicitly
+                        # allows it
+                        StrRe("(@#chan1 @#chan2|@#chan2 @#chan1) ?"),
                     ],
                 )
             elif m.command == RPL_WHOISSPECIAL: