diff --git a/CHANGELOG.md b/CHANGELOG.md
index 382946b..b91271d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,13 @@
 Changelog
 =========
 
+## TBD
+
+### Bug fixes
+
+* Avoid reading `__code__` when setting a custom delivery unless it exists
+  [#387](https://github.com/bugsnag/bugsnag-python/pull/387)
+
 ## v4.7.0 (2024-04-24)
 
 ### Enhancements
diff --git a/bugsnag/configuration.py b/bugsnag/configuration.py
index f96752b..114de35 100644
--- a/bugsnag/configuration.py
+++ b/bugsnag/configuration.py
@@ -279,7 +279,11 @@ def delivery(self, value):
             # this should be made mandatory in the next major release
             if (
                 hasattr(value, 'deliver_sessions') and
-                callable(value.deliver_sessions)
+                callable(value.deliver_sessions) and
+                # Mock objects don't allow accessing or mocking '__code__' so
+                # ensure it exists before attempting to read it
+                # __code__ should always be present in a real delivery object
+                hasattr(value.deliver_sessions, '__code__')
             ):
                 parameter_names = value.deliver_sessions.__code__.co_varnames
 
diff --git a/bugsnag/sessiontracker.py b/bugsnag/sessiontracker.py
index edb8bf2..adc3153 100644
--- a/bugsnag/sessiontracker.py
+++ b/bugsnag/sessiontracker.py
@@ -139,7 +139,10 @@ def __deliver(self, sessions: List[Dict], asynchronous=True):
 
             deliver = self.config.delivery.deliver_sessions
 
-            if 'options' in deliver.__code__.co_varnames:
+            if (
+                hasattr(deliver, '__code__') and
+                'options' in deliver.__code__.co_varnames
+            ):
                 try:
                     post_delivery_callback = self._request_tracker.new_request()
 
diff --git a/tests/test_client.py b/tests/test_client.py
index f01b858..2b7233f 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -18,6 +18,7 @@
     FeatureFlag
 )
 
+from bugsnag.delivery import Delivery
 import bugsnag.legacy as legacy
 from tests.utils import (
     BrokenDelivery,
@@ -124,6 +125,22 @@ def deliver(foo, config, payload, options={}):
         self.assertTrue(self.called)
         del self.called
 
+    # test for a regression caused by reading '__code__', which does not exist
+    # on Mock objects, nor can it be mocked
+    # see: https://github.com/bugsnag/bugsnag-python/commit/77e11747c293ba715bc764d17b49fb32918c030a#r142130768
+    def test_delivery_can_be_mocked(self):
+        delivery = Mock(spec=Delivery)
+
+        client = Client(delivery=delivery, api_key='abc')
+        client.notify(Exception('Oh no'))
+
+        assert delivery.deliver.call_count == 1
+
+        client.session_tracker.start_session()
+        client.session_tracker.send_sessions()
+
+        assert delivery.deliver_sessions.call_count == 1
+
     # Capture
 
     def test_notify_capture(self):