--- a/cubicweb/pyramid/session.py	Wed May 08 20:53:49 2019 +0200
+++ b/cubicweb/pyramid/session.py	Wed May 08 21:30:44 2019 +0200
@@ -89,7 +89,7 @@
 from contextlib import contextmanager
 
 from pyramid.compat import pickle
-from pyramid.session import SignedCookieSessionFactory
+from pyramid.session import SignedCookieSessionFactory, JSONSerializer, PickleSerializer
 
 from cubicweb import (
     Binary,
@@ -129,6 +129,24 @@
             yield cnx
 
 
+class JSONSerializerWithPickleFallback(object):
+    def __init__(self):
+        self.json = JSONSerializer()
+        self.pickle = PickleSerializer()
+
+    def dumps(self, value):
+        # maybe catch serialization errors here and keep using pickle
+        # while finding spots in your app that are not storing
+        # JSON-serializable objects, falling back to pickle
+        return self.json.dumps(value)
+
+    def loads(self, value):
+        try:
+            return self.json.loads(value)
+        except ValueError:
+            return self.pickle.loads(value)
+
+
 def CWSessionFactory(
         secret,
         cookie_name='session',
@@ -177,7 +195,7 @@
         reissue_time=reissue_time,
         hashalg=hashalg,
         salt=salt,
-        serializer=serializer)
+        serializer=serializer if serializer else JSONSerializerWithPickleFallback())
 
     class CWSession(SignedCookieSession):
         def __init__(self, request):