diff --git a/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py b/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
index b8531db17..721466d55 100644
--- a/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
+++ b/google/cloud/pubsub_v1/subscriber/_protocol/streaming_pull_manager.py
@@ -941,7 +941,12 @@ def _shutdown(self, reason: Any = None) -> None:
             assert self._leaser is not None
             self._leaser.stop()
 
-            total = len(dropped_messages) + len(
+            if dropped_messages is not None:
+                total = len(dropped_messages)
+            else:
+                total = 0
+    
+            total = total + len(
                 self._messages_on_hold._messages_on_hold
             )
             _LOGGER.debug(f"NACK-ing all not-yet-dispatched messages (total: {total}).")