--- a/autobahn/wamp/test/test_wamp_serializer.py	2024-04-13 20:34:22.000000000 +0200
+++ b/autobahn/wamp/test/test_wamp_serializer.py	2026-10-26 22:51:48.655578300 +0100
@@ -176,9 +176,10 @@
         _serializers.append(serializer.MsgPackSerializer())
         _serializers.append(serializer.MsgPackSerializer(batched=True))
 
-        # roundtrip error
-        _serializers.append(serializer.UBJSONSerializer())
-        _serializers.append(serializer.UBJSONSerializer(batched=True))
+        # roundtrip error; optional backend, may be absent
+        if hasattr(serializer, "UBJSONSerializer"):
+            _serializers.append(serializer.UBJSONSerializer())
+            _serializers.append(serializer.UBJSONSerializer(batched=True))
 
     # FIXME: implement full FlatBuffers serializer for WAMP
     # WAMP-FlatBuffers currently only supports Python 3
--- a/autobahn/twisted/test/test_tx_choosereactor.py	2024-04-13 20:34:22.000000000 +0200
+++ b/autobahn/twisted/test/test_tx_choosereactor.py	2026-10-26 22:51:48.655611213 +0100
@@ -52,16 +52,17 @@
         Patch ``sys.modules`` so that Twisted believes there is no
         installed reactor.
         """
-        old_modules = dict(sys.modules)
+        old_reactor = sys.modules["twisted.internet.reactor"]
 
-        new_modules = dict(sys.modules)
-        del new_modules["twisted.internet.reactor"]
+        # NOTE: mutate sys.modules in place instead of rebinding it; a
+        # wholesale rebind leaves stale references behind (e.g. in cbor2's
+        # encoder, breaking its module-based type hooks with KeyError).
+        del sys.modules["twisted.internet.reactor"]
 
         def _cleanup():
-            sys.modules = old_modules
+            sys.modules.setdefault("twisted.internet.reactor", old_reactor)
 
         self.addCleanup(_cleanup)
-        sys.modules = new_modules
 
     def test_unknown(self):
         """