diff --git a/.travis.yml b/.travis.yml
index a4e2390e2..9b6ef5b68 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,7 +39,7 @@ matrix:
     #
     # Flake8
     #
-    - python: "3.5"
+    - python: "3.8"
       env:
       - TOX_ENV=flake8
 
diff --git a/autobahn/wamp/message.py b/autobahn/wamp/message.py
index fa74c71f9..8461654be 100644
--- a/autobahn/wamp/message.py
+++ b/autobahn/wamp/message.py
@@ -378,6 +378,8 @@ class Message(object):
         '_correlation_uri',
         '_correlation_is_anchor',
         '_correlation_is_last',
+
+        '_router_internal',
     )
 
     def __init__(self, from_fbs=None):
@@ -393,6 +395,9 @@ def __init__(self, from_fbs=None):
         self._correlation_is_anchor = None
         self._correlation_is_last = None
 
+        # non-serialized 'internal' attributes (used by Crossbar router)
+        self._router_internal = None
+
     @property
     def correlation_id(self):
         return self._correlation_id