diff --git a/lower-constraints.txt b/lower-constraints.txt
index ba0e7d8d8d09..292a9d5e65d7 100644
--- a/lower-constraints.txt
+++ b/lower-constraints.txt
@@ -87,7 +87,7 @@ oslo.policy==1.35.0
 oslo.privsep==1.23.0
 oslo.reports==1.18.0
 oslo.rootwrap==5.8.0
-oslo.serialization==2.18.0
+oslo.serialization==2.21.1
 oslo.service==1.33.0
 oslo.upgradecheck==0.1.1
 oslo.utils==3.37.0
diff --git a/nova/rpc.py b/nova/rpc.py
index 6d7de3d67690..472698330533 100644
--- a/nova/rpc.py
+++ b/nova/rpc.py
@@ -33,6 +33,7 @@ from oslo_messaging.rpc import dispatcher
 from oslo_serialization import jsonutils
 from oslo_service import periodic_task
 from oslo_utils import importutils
+import six
 
 import nova.conf
 import nova.context
@@ -118,9 +119,27 @@ def get_allowed_exmods():
 
 
 class JsonPayloadSerializer(messaging.NoOpSerializer):
+
     @staticmethod
-    def serialize_entity(context, entity):
-        return jsonutils.to_primitive(entity, convert_instances=True)
+    def fallback(obj):
+        """Serializer fallback
+
+        This method is used to serialize an object which jsonutils.to_primitive
+        does not otherwise know how to handle.
+
+        This is mostly only needed in tests because of the use of the nova
+        CheatingSerializer fixture which keeps some non-serializable fields
+        on the RequestContext, like db_connection.
+        """
+        if isinstance(obj, nova.context.RequestContext):
+            # This matches RequestContextSerializer.serialize_context().
+            return obj.to_dict()
+        # The default fallback in jsonutils.to_primitive() is six.text_type.
+        return six.text_type(obj)
+
+    def serialize_entity(self, context, entity):
+        return jsonutils.to_primitive(entity, convert_instances=True,
+                                      fallback=self.fallback)
 
 
 class RequestContextSerializer(messaging.Serializer):
diff --git a/nova/tests/fixtures.py b/nova/tests/fixtures.py
index 6fc09b4cae5b..53e5a795124f 100644
--- a/nova/tests/fixtures.py
+++ b/nova/tests/fixtures.py
@@ -826,6 +826,15 @@ class WarningsFixture(fixtures.Fixture):
         # prevent adding violations.
         warnings.filterwarnings('error', message=".*invalid UUID.*")
 
+        # NOTE(mriedem): Avoid adding anything which tries to convert an
+        # object to a primitive which jsonutils.to_primitive() does not know
+        # how to handle (or isn't given a fallback callback).
+        warnings.filterwarnings(
+            'error',
+            message="Cannot convert <oslo_db.sqlalchemy.enginefacade"
+                    "._Default object at ",
+            category=UserWarning)
+
         self.addCleanup(warnings.resetwarnings)
 
 
diff --git a/nova/tests/unit/fake_notifier.py b/nova/tests/unit/fake_notifier.py
index 526845c85ca9..62c2fa8bb557 100644
--- a/nova/tests/unit/fake_notifier.py
+++ b/nova/tests/unit/fake_notifier.py
@@ -17,12 +17,16 @@ import functools
 import pprint
 import threading
 
+from oslo_log import log as logging
 import oslo_messaging as messaging
 from oslo_serialization import jsonutils
+from oslo_utils import excutils
 from oslo_utils import timeutils
 
 from nova import rpc
 
+LOG = logging.getLogger(__name__)
+
 
 class _Sub(object):
     """Allow a subscriber to efficiently wait for an event to occur, and
@@ -96,7 +100,11 @@ class FakeNotifier(object):
                               serializer=self._serializer)
 
     def _notify(self, priority, ctxt, event_type, payload):
-        payload = self._serializer.serialize_entity(ctxt, payload)
+        try:
+            payload = self._serializer.serialize_entity(ctxt, payload)
+        except Exception:
+            with excutils.save_and_reraise_exception():
+                LOG.error('Error serializing payload: %s', payload)
         # NOTE(sileht): simulate the kombu serializer
         # this permit to raise an exception if something have not
         # been serialized correctly
diff --git a/nova/tests/unit/test_rpc.py b/nova/tests/unit/test_rpc.py
index ef89921829be..6cbccd245f05 100644
--- a/nova/tests/unit/test_rpc.py
+++ b/nova/tests/unit/test_rpc.py
@@ -18,6 +18,7 @@ import mock
 import oslo_messaging as messaging
 from oslo_messaging.rpc import dispatcher
 from oslo_serialization import jsonutils
+import six
 
 from nova import context
 from nova import rpc
@@ -366,10 +367,20 @@ class TestRPC(test.NoDBTestCase):
 
 class TestJsonPayloadSerializer(test.NoDBTestCase):
     def test_serialize_entity(self):
+        serializer = rpc.JsonPayloadSerializer()
         with mock.patch.object(jsonutils, 'to_primitive') as mock_prim:
-            rpc.JsonPayloadSerializer.serialize_entity('context', 'entity')
+            serializer.serialize_entity('context', 'entity')
 
-        mock_prim.assert_called_once_with('entity', convert_instances=True)
+        mock_prim.assert_called_once_with('entity', convert_instances=True,
+                                          fallback=serializer.fallback)
+
+    def test_fallback(self):
+        # Convert RequestContext, should get a dict.
+        primitive = rpc.JsonPayloadSerializer.fallback(context.get_context())
+        self.assertIsInstance(primitive, dict)
+        # Convert anything else, should get a string.
+        primitive = rpc.JsonPayloadSerializer.fallback(mock.sentinel.entity)
+        self.assertIsInstance(primitive, six.text_type)
 
 
 class TestRequestContextSerializer(test.NoDBTestCase):
diff --git a/requirements.txt b/requirements.txt
index 2d22340d97fb..c782f8dd1107 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -41,7 +41,7 @@ oslo.config>=6.1.0 # Apache-2.0
 oslo.context>=2.19.2 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslo.reports>=1.18.0 # Apache-2.0
-oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0
+oslo.serialization!=2.19.1,>=2.21.1 # Apache-2.0
 oslo.upgradecheck>=0.1.1
 oslo.utils>=3.37.0 # Apache-2.0
 oslo.db>=4.40.0 # Apache-2.0